In addition to what Bob said, you should be able to toggle the wireless on that model using the Fn + F2 key combo.
But first let's make sure it even HAS the wireless card. Get out your trust phillips head screwdriver and flip the system over. There should be two removable panels held in by a single screw each. It should be the larger one. Take that off and look for something that looks like this:
If it's not under the bigger of the two panels, take the other one off. You won't hurt anything by doing so.
If you don't see something like this under either panel, then you don't have the wireless card, and that would be the source of your problems. In which case feel free to buy one from the link above. They're real simple to put in, since they only fit one way. You just need to connect the black and white antenna wires, which should be in the compartment, and there will be black and white triangles to tell you which goes where.
If you DO see a card like this, and the antennas are connected correctly, see Bob's advice about making sure the drivers are installed. You can get these off Dell's website: http://support.dell.com and just find your specific model or enter in the service tag (white sticker on the under side). There should be drivers for the wireless card there.
If that doesn't do it, your wireless card is likely bad, and will need to be replaced.