Did you read http://forums.cnet.com/7723-6121_102-326854/can-t-get-wireless-to-work-on-dell-inspiron/ ? There it was fixed.
But it should be fine automatically if you used the recovery media that came with the laptop.
I've also seen a post somewhere that said he had to press just f2 without fn-key. Then it seems some fn-lock was set.
I reimaged my sister-in-laws Dell Inspiron 1521, and the wireless card wouldn't acknowlege available networks. Reloaded drivers, tried different drivers, rolled back drivers, nothing is working. It is showing that there is wireless capability, but it just shows no networks available, while every other device in the house sees both of my hotspots, plus several neighbors.
There is no switch or function key to enable/disable wireless, though the Windows Troubleshooter keeps stating that this is the issue. I've replaced the card (Mini PCI-E) and updated driver accordingly (remove old, install new) and it is still not working.