Check out this message which explains it nicely too:
https://bitcointalk.org/index.php?topic=82213.msg906833#msg906833You can then send funds into that 2-of-3 transaction using the normal sendtoaddress/sendmany RPC commands, or the GUI (or anything that's been updated to recognize multisig addresses).
That will create a multi signature transaction.
If you want to then SPEND the funds sent in that 2-of-3 transaction... that isn't implemented yet (well, if bitcoin has all 3 private keys in it's wallet then the funds will show up in the wallet's total balance and it may be selected to be spent like any other funds received by the wallet). "We" need to implement RPC calls to implement BIP 10 or something like it.
So it looks like maybe the spend isn't there yet?
That was written much later than the post you linked.