So I've been trying to start the game for 2 days... I'm on OS X 10.8.2, and I downloaded through Steam.
First time through the installation, the game kept getting stuck at the "patch dat files..." and I looked through the forum, tried the solutions... then the problem was the "update error," which I got around by removing the "tm" sign from the file names as specified in the solution threads. Then I had a problem with the getting stuck again, where the launcher is not frozen or anything, but it just stops at random times and stops downloading the patch files. Then I rebooted, and after 2 more tries, I finally was finished with the updating @$% and FINALLY saw my login screen!
But of course, after alllll this ordeal, I try to open the game (rather than through Steam, I opened it with the direct LOTRO file (with the ring icon)), but I get: Game Error  Can't open the data files. Check that they exist and that you have permission to write them. The program will now exit. [PlatformStringID_DatFile_Error]
And the error number seems to change, always starting with 3 though.
Then I tried opening through Steam, and no luck: "MISSING EXECUTABLE." Whatever the hell this means... I thought it might be because I changed the file names, but even after adding the "TM"s back, same thing.
I just wasted a lot of time, effort and internet usage for all this...
Can anyone please help me?