Display form from an VB application in Excel

Dear All,

I have the following problem with a macro. I hope somebody may help me.

Problem:
I want to display a VBA form created in an VB application on top of an
Excel worksheet to enter some information. But when I show the form
after opening the Excel workbook, the macro automatically switches back
to the application showing this form (and not on top of the Excel
workbook).

Code:
....
'Open Excel workbook
Set xlS = clsApp.GetApplication("excel.Application")
Set xlsWB = xlS.Workbooks.Open(filename)
xlS.Visible = True
'Show form on top of that Excel workbook
frm.Show vbModeless
PROBLEM OCCURED: the macro switches back to the VB application showing
this form instead of staying with Excel
....

Purpose of the macro:
I have this VB application which provides Visual Basic Editor for
writing VBA macros. In order to import data from any Excel sheet, I
open an Excel workbook with the macro and then want to show a VBA form
on top of the opened Excel sheet. The Excel sheet itself cannot contain
any macro at all. Those Excel sheets will have the data always in
different columns so the user needs to see the Excel sheet as well as
the form in order to enter the proper column names in the form.

I would appreciate any help of you guys. MANY THANKS in advance!

Best wishes,
kuhni