You can choose slots and table for the free play. When you go to the cage, you tell them how much you want for tables. i.e., your offer has $2000 in free play, they will ask you if you want tables or slots. You didn't tell them you want $1,500 for slots and $500 for tables or whatever combination you want. They will then give you $500 worth of chips and you will use your selling sign card in the slot machine to access the other $1500.
Also, to save you a little time of having to get in the line twice, stick your card in any slot machine before going to the cage. This simply activates your card on the boat. If you do not when you go to the cage, they're going to tell you to get out of the line and stick it into a machine. So save yourself some time. It takes 2 seconds to stick it in. You do not have to play. You only have to insert it and then it'll prompt on the screen that you've been rewarded free play. After you see that message, you're good to go to the cage and get whatever money combination you want..