1) UnArchiver a light weight extractor, get it on the Mac App store from here. (It's free.)
2) That's it. You can now open .7z extension on your Mac.
Unarchiver will also open extensions like Zip, RAR, 7-zip, Tar, Gzip and Bzip2. Many older formats, such as StuffIt, DiskDoubler, LZH, ARJ and ARC if need be.