The Index Lookup puzzle is the arrow-puzzle with circles/lights that can be found inside the tower.

Its quite a bit easier than the other two puzzles outside the tower so I'd recommend just giving it a go without looking at the spoilers.


Spoilers:

• Beside each arrow is 5 circles representing 5 floors of the tower, starting from the first one with a symbol.
  
• Each floor has a symbol that matches up to the corresponding arrow lit-up for that floor.
  
• Using the circle with symbols on it (above the chest) you can determine which direction the arrow should point to match the symbol on that specific floor.