Instructions
First off you need the special patch to play on you MAC. Go and dl the Starcraft OS X Intstaller, it can be found on the blizzard support website at this link: http://us.blizzard.com/support/article.xml?locale=en_US&articleId=21149
After you downloaded the dmg, you can either get the starcraft installer with the original starcraft &broodwar on a newsgroup like, Usenext for free or if you have the CD you can install it the old fashion way. The first option allows you the mount the game on your Mac so that you don't need the CD anymore.
If you plan to just play this offline with some buddies you can use any old CD key or make up one. This will let you go on Battle.Net just to get the update for the latest Starcraft. Then you can play on the LAN with your friends. If you really want to play on Battle.Net i suggest you buy the CD keys online for $8.99. Finding CD keys posted online that haven't been band are virtually impossible.