my 90 F-150 drivers door wont unlock or open when it is very cold outside, could it be that there is moisture inside the locking mechanism? any ideas on how i could fix this?

does the key turn at all?
if so just go to local auto store and get some lock deicer and try that. if key does turn then u might have borken the lock mech.

no when it is really cold the key will not turn to unlock the door, but when its not very cold the lock works fine.

Sure sounds like it could be frozen, if the problem only occurs when it is very cold (I take it you mean 32 degrees F or below )

The easiest way to fix this problem is to stop locking your door in the winter. Voila.

Or you can try to displace whatever moisture is in the lock with lubricant such as a light oil.