To get the next locked car (only next car) have your finger by the Select button and go onto a unlocked car then scroll down one (works on paid and free version) car (locked car) and in mid-scroll press Select.
Non Unlocked Cars
Whatever car you have, it'll only work for the car right after the one you already have, for example, when using the black car. You swipe down to the general and press Select very quickly afterwards. When you unlock general, you do the same to use the next car. Etc.
Unlock all guns
First you have to go to zombie highway. Goto a game center sign out, play a match of zombie highway on any level, get ten kills, then go back to game center sign in then go to zombie highway go to the gun shop and you will have all guns.
Use cars you don't have
Find a car you have, then scroll down really fast to the one you don't have and click Select.