What do you mean with:
"it doesn't take it?"
If you use the type of refilling that Wall*Mart offers, you have to put the hose with the pressure gage on the R134A can, turn in the screw that punctures the can, down.
To release the gas out of the can, you then have to loosen (unscrew) the can from the hose, but only about 1/2 to 1 revolution. You should feel that the can gets lighter. (hold can upside down)
Only do that with the motor running, and the A/C full performance working. You won't get any gas into the system unless the motor is not running.
Make sure that the connector on the car is properly working.
You can use a screw driver and push the valve indide the connector of the A/C system.
!!!!!! W A R N I N G !!!!!
BE VERY CAREFUL WITH THE FREON! YOU GET A SHOT OF FREON IN YOUR EYE, YOU CAN GET BLIND!!!!!!!!!
An other problem might occur if you overfilled the A/C - I THINK they have something like a safety switch which disables the compressor, just to prevent any damage to the system. But I am not 100 % sure on that with the Mustangs. My SAAB had one.