How to access Add-in module from VBA macro

I have created an Excel Add-in using VS2010 using C#. Inside the ThisAddIn class, along with the Startup and Shutdown events, I have added a function, AddMapping - which essentially sets up an XML mapping to a worksheet. I want to trigger this function from inside a VBA macro inside a loaded spreadheet - how do I access this function in the Add-in module - I do not see the Add-In module in the Project Explorer when VBA is up - as per this

I'm very new to Add-ins - I believe this has created a COM add-in - not sure if that has anything to do with it.

Thanks for any help