BearShare has only ever been designed to run on Windows. If you have Windows running on your mac then that's fine. Personally I use Parallels to run several windows. There's also the open source & free Wine. Alternatively can just install Windows either using Bootcamp or alone on another drive or partition (this last method means you need to use Windows specific drivers & equipment, etc.) My windows installations kept corrupting so I gave up on Bootcamp. One advantage of both Wine & Parallels is can run at same time as OSX, & even have several Window's OS's running simultaneously. It also has snapshot backup system in case of any issues. Easy drag & drop between the OS's.

To download BearShare, go to the BearShare section of the forum, you will see links to where to get it.