Application.RegisterXLL

This loads an XLL resource and automatically registers the functions and commands.
This will not add an "OPEN" entry into the registry and the file is only loaded for this current session.
This will not add the item to the Application.Addins collection.

Application.RegisterXLL("C:\temp\myFile.xll") 

This loads the dll and returns True if successfully loaded

Dim breturn as Boolean 
breturn = Application.RegisterXLL "mylibrary.xll" 

Application.RegisteredFunctions