Complete the puzzle TLPTHC in I/O STREAM section. This is one puzzle down and two puzzles to the right of the first puzzle in that section. Text solution:
ULLS USSS DRRR RSSS US
(U=Up, L=Left, R=Right, D=Down)
Note that you also have to beat the second level called SNCHRNZ to unlock the third level. Text solution:
LUUR RRRR UUDR RRDD
(U=Up, L=Left, R=Right, D=Down)
Full achievement video walkthrough. This puzzle can be seen at 3:59: