Through Steam.
When you installed the game, it should have installed Steam.
Unless you pirated the game, in which case, find a torrent with the patches already installed.
Received it today on dvd disc. Installed but game has not updated yet, still on the nov 13 update only. Restarted pc, and restarted steam numerous times.
[Edited by m8tal, 12/22/2009 1:43:12 PM]
Figured out the trainers refer to the trainer patch and not a steam patch. So the latest trainer worked for me but if everyone has the same copy of COD MW2 on steam I don't see why the previous trainers are still available if they don't work. I'm coming from the standpoint that previously playing Empire total war, there were different patched trainers for empire patches.
Previous trainers are still available for those of us who stop MW2 from updating by running Steam offline.