I too use a Mac and there are a few programs that open/join compressed files. The two most common are UnRarX and AJoiner, both are available through www.versiontracker.com . I have used both on Mac OS X 10.3 10.4 and 10.5, on PowerPC and Intel based Macs.
UnRarX
http://www.versiontracker.com/dyn/moreinfo/macosx/15272
AJoiner
http://www.versiontracker.com/dyn/moreinfo/macosx/17123
If you use a Windows machine use WinRar, again check with www.versiontracker.com for the current version.
http://www.versiontracker.com/php/qs.php?mode=basic&action=search&str=winrar&srchArea=all&submit=Go
When you have problems installing WinRar on your Windows PC remember what a friend of mine says:
Since 1984 there have been three kinds of computers:
those that are large and hard to use,
those that are small and hard to use,
and those known as Macintosh.