64-bit Office apps--including Project 2010--do not support COM add-ins, which are 32-bit. To use the 64-bit environment, COM add-ins must be ported to a 64-bit add-in by using the Office Development Tools in Visual Studio 2010 (often referred to as Visual Studio Tools for Office, or VSTO).
There is no direct conversion tool for porting 32-bit COM add-ins to VSTO for 64-bit Office apps. The amount of time to port a COM add-in can vary greatly, depending on the functionality and Project version for which it was designed. A simple add-in designed for 32-bit Project 2010 might take a few hours; something like Solution Starter: Scrum for Project 2003 (although I haven’t done a port) might take several weeks and require a complete redesign, because it uses features that are no longer available in Project and Excel 2003 – and it was written in VB6.
· VSTO in VS 2010 supports only Office 2007 and 2010.
· Carefully check the types when you convert the code (int, long, etc.).
· When doing the port, you can use compiler constants to make the code usable in both 32-bit and 64-bit Office apps. See Tables of VBA Object Model Changes for a VBA example, and for a comparison of types in VB and C#.
See also (VBA-related, not specifically for add-ins) Working with VBA in Office 2010 (32-bit) and Office 2010 (64-bit).
You might also want to check out Add-In Express 2010; see, for example, Converting existing Office COM add-in projects to Add-in Express.