Calling a Add-in function from Excel's VBA

I am using an Excel Add-in for an Erlangs: http://abstractmicro.com/erlang开发者_StackOverflow/helppages/ref-erlbblockage.htm

I try to call the Erlang-B function within the Add-in from within VBA thus:

Function Erl(Erlangs As Double, Capacity As Double)
    Erl = Application.WorksheetFunction.ErlbBlockage(Capacity, Erlangs)
End Function

...but it doesn't work. I get #VALUE! returned in the Excel cell. I think it is because the function is not part of standard Excel (it is in the Add-in). So how do I call it?

Try setting a reference in your VBA project to the Erlang Addin
VBE-->Tools-->References check A_Erlang

Then you can call the functions in the addin directly from VBA

Myvar = ErlbBlockage(capacity, erlangs)