i fitted a cheap £10 kit to my focus. real easy to do as well. when i bought mine i could only use they key in the doors and the buttons didnt work but try this first (if you have key with buttons on):
jump in car and insert key
turn key to pos. 2 on lock and back to 0 again 3 times and you should hear a chime.
press any button on key and you should hear another chime
key should not lock/unlock car from buttons. if not key is knackered. cheaper to buy a cheap c/l kit and splice into wiring.