can I work yet????

Hi,
I am from spain and I am attending school in the US, i just applied a few months ago for an off campus working permit and on the 28th I checked my status online and it said it had been approved but and that they had order my card and this would be sent as soon as it was ready. My question is, since I have been approved do I need to have the card in hand in order to be able to work and if so how long do they usually take to send the cards?
thanks