First, you haven't installed the add-in as an add-in. Double clicking on an
add-in
in Explorer opens the workbook, but not as an add-in, so it won't show up in the list.
Thanks Jon. However, I think you misunderstood my question. I did not
double click on an add-in. The fact that I put it in the XLSTART seems to be
sufficient for Excel to realize that this Add-in should be loaded when Excel
is launched.

Second, as Fredrik points out, opening Excel via Automation avoids opening any of the installed add-ins. But you can open the add-in file using
xlApp.Workbooks.Open(<add-in full name)
Again, I am not trying to open the Add-in in Excel. Rather I am trying to
open a different workbook and in that workbook I want to use the functions
defined in my add-in.