I made three separate accounts and opted them all in for the beta, one with windows and two with mac specs, but still no luck.
From what I've heard, mac users do not get invited to the first phase of beta. (That is unless you get invited cause you attended blizzcon) And all invites in the first wave have already been sent out, so if you didn't receive one yet, you're out of luck. Next chance is in roughly 2 weeks according to a bnet moderator.
And about whether or not nvidia 9400 can handle it or not, no one knows yet unless someone in the beta has tried the PC version. It's probable that it will run it at very low settings, but huge armies clashing might cause spikes. I'm hoping it will suffice since I have a 9400 MBP myself.