1) Download the application called RAR at
http://files3.rarlab.com/rar/rarosx-3.5.1.tar.gz
2) decompress that file and there'll be a folder called "rar".
3) in that folder there's a file called "rar" (with no quotes or extension)
4) In Finder, go to Go > Go To Folder, and type in "/bin".
5) drag the FILE called "rar" in to that bin folder. It will ask you for your password, so go type it in and continue.
6) open the terminal and go to the directory that has the file or files you want to rar.
7A) once in the folder housing the files you want to rar, type this:
rar a FileName.rar
7B) Alternatively, you can specify which file or folder inside the folder you're in, you want to rar:
rar a FileName.rar originalfilename.whatever
8) to make a multi file rar archive, type the following instead of the above and replace where you see "####" with the file size you want each part to be, in kilobytes (kb).
rar a -v