How can I play Dungeon Defenders cross-platform PC-PS3?

I don't believe this is actually currently supported. It's listed as a feature, but it's also listed as "coming soon."

In a reply to a forum thread, one of the developers of the game stated:

We will post as we have more information. Cross-platform is a feature that we really want to implement. We also want to get fixes and improvements via patches to Console users. Unfortunately many things are simply out of our hands. We are working day and night on our product and hope you guys can bare with us.

This was the freshest data I could get, and it's dated November of 2011.

They've also mentioned that patching the console versions is a costly process ($10,000 according to other replies from the devs in that thread).

It's hard to say when the support will be there, as that's speculative and not really authoritatively answerable.