Jamie just sent me the links to the latest version of the reflector addin:
 Managed Add-Ins / Reflector Add-In
 Code Project VS.NET Add-Ins Competition
A few hints to get you started....
1) You can right click on any Windows Forms project and select 'Managed Add-In..' to run it as an add-in.
2) In VS.NET 2003 you will find a C# Managed Add-Ins project wizard (have a look as the app.config file to see what it does).
3) You will have to click the save icon on the 'Add-Ins Toolbox' to have add-ins persisted.
4) You will find the Reflector Add-In solution available on the 'Start/All Programs' menu (be sure to read the readme).