Take the Mexico Ave Cutoff, entrances are across from the Cargo (UPS, Fedex) areas. It's a small driveway into the chainlink fenced parking lot. Look for the very small 'employee lot' sign that is only about 3 feet high. I was using that parking from December to March and the gate arm was always up. There is supposed to be a monthly charge but I was lucky to go thru the open gate each time and use it for free. There is not even a security patrolling the lot or anything so I had no problems using it.
If I remember correctly, the "prison" buses run about every 15 minutes until 10pm and it runs every 30 minutes from then on.