There are two ways to download RatBZEdit. The way for most users is to use the official download page here. You can also download the latest development version from subversion. You can do this with these commands (in linux):

$ cd
$ svn co

Do not include the dollar-signs in the commands. RatBZEdit is now saved in a directory named "trunk" in your home directory. I don't know what the commands are in Mac or Windows.