It was originally not an option on the inspire when it 1st came out, there was an AT&T over the air update a couple months later (april maybe) that allowed you this option.
Are you using the phone on something other than ATT? If you do settings about phone is it version 2.2xxx or 2.3xxx?
and check security settings and app development settings too, I know htc moved something to one or both those places with their gingerbread update, I can't remember if it was this.