#1 - Sept. 5, 2014, 9:18 p.m.
Once the zone is open, Nexus-Lord Donjon Rade (in Stormwind, Orgrimmar, Shattrath, Dalaran, Vale Shrines, or your garrison) will have a chat option to teleport you into Blackrock Foundry. Assemble your group, set the raid difficulty to Mythic, and then use that option to enter.
We apologize for the mess with testing today. We go through a lot of data and hotfix acrobatics to test bosses often in an order the basic game would never permit or support, and sometimes things break. We're going to be doing one more round of restarts to pick up needed hotfixes, and then this should work.
Please refrain from using any silly scaled-up items (or at least let us know in your feedback if your raid was doing so) - we're in the process of fixing our scaling tech so that old items will be ignored, but in the meantime, let's try to keep feedback constructive. Uploading and linking to a log parse at Warcraft Logs, or any similar site that can handle 6.0 logs, would also be quite helpful.