You can grab the instance of the Acad application using this bit of code
Public Function GetAutoCAD() As Object
' Get a reference to AutoCAD
Dim RefObj As Object
On Error GoTo EoF
Set RefObj = GetObject(, "AutoCAD.Application")
Set GetAutoCAD = RefObj
EoF:
End Function
In your application create a global variable to hold the return value. If you don't want to utilize a global var, you can just grab the AutoCAD instance whenever you need it. Beware though .. if more than one session is open it may grab the wrong one.