Just to clarify, here's the process:
1) Post here signing up for an available playtest, as I'm about to:
Sign me up for Poison Ivy, please!
2) The Lead Designer of the design you signed up for will then pm you the latest card (Sir Galahad in this case). However, if it's a public design thread, this is unnecessary and doesn't need to happen (so don't worry about it, Sir G). If it's a future, private design thread, you'll have no other way of seeing what to playtest, though.
3) After you're done with the playtest, you need to get it back to the Lead Designer. If the thread is public, you can just post it in the thread. If the thread is private, you'll need to use a pm or email to the lead designer.