For those who wants to participate in public test realm, you need to update your client version to 3.1.0. You can read the 3.1.0 patch note at official Worldofwarcraft.com site.For download, as usual WowWiki.com is the best source for linking to collections of site that provide patch downloading in their servers. Check out wowwiki PTR patch mirrors page. Below are several link you can use instantly to download your patch.
Wow wotlk Patch 3.1.0 - Patch notes
Wow wotlk Patch 3.1.0 - Patch Download - Mirror
