If you are running Windows on your Mac, then all is good. Download Better BearShare OR check File_Girls BearShare adaption & update.
BearShare was only ever made to run on Windows operating system, never Linux or Mac OS.

How to run windows on your mac?

4 options I can think of:

1. Mac's Bootcamp utility will help you install Windows & install the drivers to run mac hardware whilst booted into Windows.

2. Install Windows on mac, but use windows drivers & regular pc mouse, keyboard, etc.

3. Use a virtual pc software which allows Windows to run inside a window interface on your computer. Parallels is what I use & is easy to open. I like this option because it means I do not need to reboot into windows, simply open parallels. VMware is another good vpc. I've heard Parallels is a little faster. I run windows 2k, xp & 7. Parallels also supports Linux. http://en.wikipedia.org/wiki/Parallels_Desktop_for_Mac

4. Use Wine. This allows you to run Windows programs whilst booted into Mac OSX. This is not a virtual pc software like (3.) above. And unlike the vpc's, Wine is free. Wine (software) - Wikipedia, the free encyclopedia. Direct site is here http://winebottler.kronenberg.org/. Well worth trying IMO.