If you have installed Parallels and then the Windows version of RootsMagic on your Mac, use File>Open on your current .rmgc database file, or use File>Restore if you have the .rmgb backup. I do believe you need to move the files over to your Windows side.
Thanks, Renee. File>Open didn't work, but File>Restore did. I also had to relink my media files, but that didn't take long. I run Parallels in Coherence mode, so I didn't need to move any files. At least, I assume Coherence is the reason I didn't have to.