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Okay, so, me and a friend of mine are trying to play SS2 multiplayer and there's been a couple issues.
Now, I'm using a PC and she's using a Mac, so that may be the issue right there, but I don't have enough computer knowledge to know whether that's the case or not.
The issue we run into is that whenever we try to board the Van Braun the loading screen appears, and the game crashes. Before we board everything is fine, chat even works, but once on the ship the game runs into an error and shuts down. We've tried using her IP instead of mine, but when we try that I can't even join the game.
Sorry if the question is vague, but I have little computer knowledge and am hoping someone here had a similar problem they found a solution to, thanks.
Multiplayer is not very stable at the best of times, unfortunately. Since that crash is at the point at which the players should 'meet' for the first time it will be a synchronisation issue between the two computers. In a windows to windows environment the most likely reason would be that one person is using mods while the other isn't (or using different mods), so make sure that the installs and mods are as identical as possible.

Sadly that may not be the issue here though. It may well be an inherent problem where Mac and Win are not compatible for MP, we do know that Mac is doing something different from the windows version because it occasionally corrupts the breath config resulting in the instadrown bug, which does not happen on win. Anything like that is a potential problem with MP.
still, it would be interesting to see the crashdumps - if you can, open your SS2 folder (both of you, if possible), locate shock2.log and crash.dmp, use winzip or similar to compress the files, upload them to mediafire or sendspace and post the links here.