couldn't find (one's) way out of a paper bag
Particularly dim-witted, unintelligent, or incompetent; unable to do something basic, simple, or straightforward. A: "How's the new intern working out?" B: "He's a nice kid, but he couldn't find his way out of a paper bag!" Jeff's new boyfriend sure says some dumb things. The dude probably couldn't find his way out of a paper bag.