If you have a key like the one in the picture, which matches your description of the key follow the below directions.
To program the fob perform the following:
1. Put key in ignition and turn to position 1 for no more than 5 seconds.
2. Remove key
3. Press and hold the unlock button and press the lock button 3 times. The led on the fob should blink a number of times and the doors should all lock.
4. If you have more than one fob to program start at step 3.