How to diagnose a COM addin that fails to load?

I am currently working on an open-source COM addin that seems to work
fine under Excel 2003.
(in case anybody interested, it's http://sourceforge.net/projects/lokad
)

But when I try the very same COM addin (that is supposed to add a
custom command bar in Excel), it fails somehow, and in the COM addin
menu of Excel 2007, I get the message "Failed to load the
addin" (approximate translation of a French error message).

I am a bit at a loss because I do not know how to diagnose this
problem. Does anyone know how can I get the full error message that
would pinpoint the exact issue?

Thanks in advance,
Joannès
http://www.lokad.com sales forecasting