Well, you appear to be stuck in the location I am. I just read someone elses attempt to explain this room, but the explaination was vague at best. Here is what I understand, so far.
Assume the back of the room is where you first entered the room, the port side of the room is to your left when facing the front of the room, the starboard side of the room is to right when facing the front of the room. I number the beams from back to front, so the first beam starboard is #1 and the last beam starboard is #4. I also will assume you have the key and are standing on the beam outside the opening where you got the key; this would be starboard beam #4
1. Jump from the starboard beam #4 to starboard beam #2.
2. Climb the vertical portion of starboard beam #2 and jump to the vertical portion of starboard beam #1.
3. Jump from the vertical portion of starboard beam #1 to the horizontal crack in the back wall and shimmy to the right until you can stand on port beam #1.
This is where it gets vague. Here is a direct quote from another posting:
"...drop near the water, whip back across the room, jump along the wall, cross the room again, jump and climb up, and figure out how to get to the flowerpot platform /alcove."
This makes me think you need to get to the lower level of beams on the port side and then find a way to whip swing from the lower port side of the room to the the lower starboard side of the room. Then you do jumps from the back of the lower starboard side to the front of the lower starboard side. Once at the front of the lower starboard side you cross to the front of the lower port side somehow (another whip swing?) and then jump and climb to get to the front upper port side.