In the spirit of sharing things I wish someone had told me when I started with VSTO, I posted here an entry which describes what to do if you want to use satellite assemblies (i.e. reference pre-built dlls) in your VSTO Excel 2003 add-in project.
If you just follow the msdn Walkthrough on Deploying Visual Studio 2005 Tools for Office Second Edition Solutions Using Windows Installer (super helpful page), and simply add your dll to your add-in project, you will experience the following issue: your add-in will work perfectly on your development machine, the installer will run without a complaint, but your dll will be disabled, and you will get no error message or warning as to what is going on.
I struggled to figure out what the issue was, and after some (lengthy) trial and error, found out the origin of the problem: you need to grant security trust to every dll referenced by your add-in. Once you know that, it's pretty easy to do, but if you don't you may waste quite some time resolving the problem!
Cheers, and happy coding,
Mathias