Complete the puzzle MNL RMT CNTRL in MANUAL PAGES section. This is two puzzles to the right of the very first puzzle. Text solution:
RRRL UUUU UDLL RUUD LLRD DDUU UUDL LLRD DURR LUUD RRLD DSSS
(U=Up, L=Left, R=Right, D=Down)
Full achievement video walkthrough. This puzzle can be seen at 2:14: