We just updated to the latest version of XenForo and v2.2 included a lot of updates so a few things might look off at the moment. We're working through them and please let us know if you spot something. Thanks!
You first need to go into any lobby you can enter. Then go to network settings. Somwhere in the list that comes up is an option for multi-platform. Set it to true. I don't know why you need to enter a lobby first to get to this setting but as it applies to the player rather than the lobby.
The setting gets saved to the player profile I think.
When you enter a lobby you can change 'multi platform' into yes or no.
I've played one game today against a friend of mine who owns the PC version (I have the PS2 version) and it played like a dream. We live in the same city and both have high end ADSL connections. Absolutely no lag, none, nothing!
Also no half time connections and stuff, just plain good football.
We met in English/French, common friendly lobby. It's there in the middle.