If you have a dipstick then you have the 4 speed (CD4E) box. Haynes says add fluid through the tube. The snag is, levels are supposed to be checked when the fluid is hot, i.e. equivalent to a 10 min drive (15 in cold weather). The selector then has to be moved through the range and back to Park to check the dipstick with the engine idling. You're a bit stumped if you can't get out of Park...
As PURPLE says above, you should be able to unlock the drive with your key via the hole under the plug alongside the selector.
Edit: I am not suggesting that you do this to make it possible to drive the car, merely to move the selector!
I am no mechanic but maybe trickle a bit at a time down the tube and keep trying the shift and checking with the dipstick. Given that the level is supposed to be checked hot, err on the side of caution re level and be careful not to overfill. If the dipstick level gets close to Max and you still can't get out of Park then you've got other problems, consult a specialist. Not a bad idea whatever happens as you shouldn't be losing fluid, certainly not to the extent that the box locks up.
You are applying the foot-brake before trying to get out of Park...? (Sorry)