James,
I can offer no idea on how/why you lost it in the first place. I've one more scenario to suggest before I consider myself to have bottomed out.

In the .NET 2.0 command prompt, run devenv /ResetAddin against the Business Layer Generator. (use devenv /? to get an overview).

Clear the .addin file from each of the locations you put it in.

Start VS 2005. Then shut it down.

Add the .addin file to the first location it's looked for and start VS 2005. If the tool doesn't load (become available in the tools menu) then shut VS down, add the .addin file to the next location. Start up VS and verify.

I know this sounds ... strange but it's worked for me before.

--dan