You can't open .jar files on a Mac in the same way as on Windows. Instead, you'll need to use this method as detailed here: viewtopic.php?f=25&t=53398
If you're not into using the command line/terminal, then I'd recommend using Minecrafter, which simplifies the process. Hope this helps, though please do attempt using the forum search next time, as there are tons of threads on this.
If you're not into using the command line/terminal, then I'd recommend using Minecrafter, which simplifies the process. Hope this helps, though please do attempt using the forum search next time, as there are tons of threads on this.