Registry Keys

Terminology

Not Installed - the ".xlam" or ".xla" file has been removed from the computer or shared drive.
Available - the ".xlam" or ".xla" file exists on the computer or shared drive.
Not Ticked - the add-in is available but has not been loaded in to Excel.
Loaded - the add-in is ticked and provides additional functionality.

Add-in Manager Key

The Add-in Manager key indicates that an add-in appears on the list in the Add-ins dialog box.
There can be entries here that point to files that are Not Installed.
This is updated when Excel closes.

Excel 365 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager 
Excel 2021 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager 
Excel 2019 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager 
Excel 2016 - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Add-in Manager

Any add-ins that have been added under the "Options" key and then manually unticked by the user are removed from the "Options" key and added here.

Options Key

The Options key indicates that an add-in is installed and will be Loaded the next time Excel opens.
This is updated when Excel closes.
If an add-in has just been ticked by the user, the registry entry will be moved from the Add-in Manager part of the registry to the Options" part of the registry when Excel closes.

Excel 365 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options 
Excel 2021 - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Options 
Excel 2019 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options 
Excel 2016 - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Options

If the full path is not specified then the default folder location is "C:\Users\ "username" \AppData\Roaming\Microsoft\AddIns"

WScript

Windows has 2 VB Script processes.
WScript which has a GUI and is the default. The other one is called CScript and is Console based.
When you double click a .VBS file the WScript processes will run.

Installation and Automatic Loading

If you want an Excel add-in to appear automatically the next time they open their Excel you need to add an entry under the Options Key.
It is quite common for installers to create a registry key in this location.
The following script is VB Script that will add the corresponding registry entry under Options.

Const EXCEL_ADDIN_PATH_BEFORE = "HKCU\Software\Microsoft\Office\" 
Const EXCEL_ADDIN_PATH_AFTER = "\Excel\Options\" 
Const ADDIN_REG_VALUE = """C:\Program Files (x86)\Better Solutions Limited\MyExceAddin.xlam""" 

Function RegistrationKeyExists(key) 
    On Error Resume Next 
    objShell.RegRead (key) 
    If Err = 0 Then 
        RegistrationKeyExists = True 
    Else 
        RegistrationKeyExists = False 
    End If 
End Function 

Function RegistrationKeyValueGet(name) 
    On Error Resume Next 
    Dim value 
    value = objShell.RegRead(name) 
    If Err = 0 Then 
        RegistrationKeyValueGet = value 
    Else 
        RegistrationKeyValueGet = "" 
    End If 
End Function 

Function RegistrationKeyValueSet(name, value) 
    On Error Resume Next 
    objShell.RegWrite name, value, "REG_SZ" 
    If Err = 0 Then 
        RegistrationKeyValueSet = True 
    Else 
        RegistrationKeyValueSet = False 
    End If 
End Function 

Dim EXCEL_ADDIN_PATH As String 
Dim versions As Variant 
Dim officeval As Variant 
Dim objShell As Object 
Dim count As Integer 
Dim value As String 
Dim exists As Boolean 
Dim success As Boolean 

Set objShell = CreateObject("WScript.Shell") 
WScript.Echo "Excel Add-in Registration Tool" 

versions = Array("12.0", "14.0", "15.0", "16.0") 

For Each officeval In versions 
    EXCEL_ADDIN_PATH = EXCEL_ADDIN_PATH_BEFORE & officeval & EXCEL_ADDIN_PATH_AFTER 

    WScript.Echo "Checking Office Version - " & officeval 

    If (RegistrationKeyExists(EXCEL_ADDIN_PATH) = True) Then 
        count = 0 
        exists = False 
        value = RegistrationKeyValueGet(EXCEL_ADDIN_PATH & "OPEN") 

        While (Len(value) > 0 And Not exists) 
            count = count + 1 
            value = RegistrationKeyValueGet(EXCEL_ADDIN_PATH & "OPEN" & count) 
            If value = ADDIN_REG_VALUE Then 
                exists = True 
            End If 
        Wend 

        If (exists = True) Then 
            WScript.Echo "Office Version - " & officeval & " Already Registered" 
        Else 
            If (count > 0) Then 
                success = RegistrationKeyValueSet(EXCEL_ADDIN_PATH & "OPEN" & count, ADDIN_REG_VALUE) 
            Else 
                success = RegistrationKeyValueSet(EXCEL_ADDIN_PATH & "OPEN", ADDIN_REG_VALUE) 
            End If 
            If (success = True) Then 
                WScript.Echo "Office Version - " & officeval & " Registration Complete" 
            Else 
                WScript.Echo "Office Version - " & officeval & " Unable to Register Add-In" 
            End If 
        End If 
    Else 
        WScript.Echo "Office Version - " & officeval & " Not Found" 
    End If 
Next 
Set objShell = Nothing

HKEY_LOCAL_MACHINE

HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Excel\Addins\

Add-ins installed into local machine are also displayed in the COM add-ins dialog box.