You use the hatch on the top-it opens by splitting down the middle. (At least according to my Kenner TIE..) I guess you would use tubes like mentioned before but there probably would have been an X-wing type ladder if there had ever been the need to show TIE pilots taking off or landing.
These were simply meant to be space cannon fodder. No shields, no advanced weapons, no hyperdrive, no real life support capability and total disregard for the pilots.