Magnetic strip readers can be hooked up so that the computer responds to them just as though you typed the information stored in the strip directly from the keyboard. If you're hooked up in that fashion, then:
Put the cursor in the desired FMP field
Scan the card
and you'll see the info stored in the magnetic strip in your field. If the information is encrypted in any way, you'll see it encrypted and then you'll have to come up with a way to remove the encryption which may or may not be impossible.
We once had one of our DB's set up so that scanning the strip on the back of a CA driver's license automatically entered their DB number into a field. The same hardware would read any credit/debit card in my wallet in the same fashion.