As far as I remember and know, there was at no time a GetCurrentSheet method on a ModelDoc2 object, it is, in fact, a method of the DrawingDoc type.
So, reading your code, I would suppose that ActiveDoc is not a DrawingDoc. To be sure, simply use:
Dim swApp as SldWorks
Dim swDrawing as DrawingDoc
Dim swDocument as ModelDoc2
Set swApp = Application.SldWorks
Set swDocument = swApp.ActiveDoc
Set swDrawing = swDocument
If swDrawing is Nothing Then
MsgBox "No valid drawing doc!"
Exit Sub
End If
Or
Dim swApp as SldWorks
Dim swDrawing as DrawingDoc
Dim swDocument as ModelDoc2
Set swApp = Application.SldWorks
Set swDocument = swApp.ActiveDoc
If swDocument.GetType() <> swDocumentTypes_e.swDocDRAWING Then
MsgBox "No valid drawing doc!"
Exit Sub
Else
Set swDrawing = swDocument
End If
And you know if this is the case. I would also recommend you to check the following additional information from the API documentation:
http://help.solidworks.com/2013/English/api/sldworksapi/Get_and_Set_Sheet_Properties_Example_VB.htm