The next owl room is the secret you get after talking to all four owls, originally there was a glitch where the room was accessible the way he entered it, but it was patched.
If you are having trouble getting into the secret door, it's mostly likely because you haven't done the prerequisite task of speaking to the 4 other owls found throughout the rest of the game. Once you've done that, the secret door will open upon following the instructions he says in the video.
I don't understand the letter for the code to get the anticube. What those letters means? R(right?), RT(¿?), L(left?), LT (left¿?¿), U (¿?), D (¿?), A (¿?). I play with a xbox controller on a pc.