A question on automating Excel from a C++ program

[Smasher/Devourer]

I've got an MFC program that I wrote using Visual C++ 6. I'd like that program to be able to modify an existing MS Excel spreadsheet by opening that spreadsheet and passing arguments to a VB macro stored within it. If it matters, the macro has the form:

Sub AddItem(ItemDesc As String, ItemCount As Integer)
    ' ... lots of stuff happens ... 
End Sub

I haven't done anything like this before so am kind of at a loss as to where to begin. I don't need a lot of details; basically I just would like someone to tell me what I should start reading to figure out how to send commands to Excel from a C++ program. Thanks!