Once you've finished downloading the PTR client, you should find it nestled within a folder called WoWTest inside your World of Warcraft folder. It's pretty much the same as the normal World of Warcraft application but with a bunch of extra files that include the changes to the game. You shouldn't have trouble identifying the client as it will be clearly labeled 'TEST', just so you don't confuse the two (those Blizzard folks are so clever!). When you log on, you can access the TTR by toggling the "Select Realm" option which will open up a selection window with two tabs: Test Server and Tournament. Simply toggle the Tournament tab and you will see the Realm named "Arena Tournament" -- select it, press Okay, and you should be good to go. If you have trouble logging in, note that the PTRs remember your password around the time when it went live, so if you changed your password within the last few weeks, try using your old password. Have fun with on the tournament server!