I had a issue where I didn't know how in the heck to debug PainlessSVN in my new Windows Ultimate x64. Thankfully, I found the answer after only a couple Google queries.

MMC 3.0 comes in both x32 and x64 versions. Normally, I would call the mmc.exe host with just C:\Windows\System32\mmc.exe in the debug tab of the project properties. I found out that I needed to append a -32 parameter at the end of this path. Here is how it is setup in the tab:

That command-line argument that I highlighted in red is very important.

I found the answer in this blog post: http://blogs.technet.com/askperf/archive/2008/04/25/controlling-your-mmc-snap-ins-on-64-bit-operating-systems.aspx