Calling Functions from Third-Party Add-Ins

How do I use a function from a Third-Party Add-In in my VBA code? The Third-Party Add-In functions are only visible from the Object Browser and only when I have the .xla file highlighted, but I don't know how to call them. For example, I tried:

Call HyperionPillarAddIn.modPillarMenuActions.OpenPillarFile("E:\SCG\Client Files\ClientName\Client Forecast.PLN", "username", "password")

This does not work - what is the syntax to utilize the functions from an .xla?

Thanks,

Shannon