To play online both players need to have the same version of the game. If there is an online option for the Mac version then you'll need to find someone else with the Mac version to play with. But since the servers are down, you'll need to also figure out if you can use Hamchi instead.
There is no lack of files available for Mac users it is mainly the lack of creation tools.
Everything available to the PC user as finished projects is also for the Mac. The only special trick Mac users need to know is how to unzip files.