That's correct Fab, you need to bring a friend and have that person bet the pass line on his or her station and you bet the don't pass line with your freeplay on your craps station (or bet two separate stations yourself if the casino allows it). If a twelve is rolled on the come out roll (or snake eyes instead in some jurisdictions) the pass line bet is lost, as Alan stated, and the Don't Pass bet is a push.
For large bets, some players like to put another hedge bet on the hard 12 horn bet which is 1/30th the size of the come bet to insure against midnight (6-6) getting rolled and losing the pass line bet.
If you don't have a friend available and the casino only allows one station per person (and the craps machine is on standard settings), you can wash your free play minus a 5% vig on the winning bet by betting your X dollars of free play on the Lay Bet on the number 4 and a bet of X/2 dollars of live money with a buy bet on the number 4 (same goes for the number 10).
You will also need to place a minimum bet (usually a buck) on the pass line or don't pass line if no one else is at the station besides you or the machine will not roll the dice.