Term for person who forgets directions or routes

Solution 1:

Normally I just say someone (me, let's say, just hypothetically) has "a bad (terrible) sense of direction."

Apparently a more serious condition exists that causes sufferers to get lost even on common paths and turned around in their own houses. This is called developmental topographical disorientation and is described in this news feature.

Solution 2:

The traditional phrase used is that these people have no sense of direction. But ajk's answer of "directionally challenged" is a great way to make this phrase into an adjective.

Solution 3:

In my family, we call those people directionally challenged.