OK, don't worry, I solved it. I should have applied the advice given in
the article you pointed me to for this problem as well:

"Note that a workbook is added prior to calling the Add method in this
example. The Add-in Manager in Excel is not available unless there is
at least one visible workbook open. Without this line of code, you
receive one of the following error messages:
Run-time error '1004':
Unable to get the Add property of the AddIns class
-or-
Run-time error '1004':
Add method of addins class failed. "

http://support.microsoft.com/kb/280290

So all I did is I first opened my workbook and then set calculation to
manual, works like a charm :-)