First of all the USB only provides 5 volts. Not 12 volts. The only way you can get 12 volts for your 12v LED is to add a DC-DC converter to convert 5 volts to 12 volts.
Secondly, a USB port is only guaranteed to provide 100 ma of current before enumeration. Enumeration is a complicated process that all intelligent devices have to go through to get connected to the USB. Part of the enumeration process is where the device tells the USB how much power it really needs. If the USB agrees, then and only then can the device begin drawing the higher current, up to 500 ma. Now some computers don't enforce the enumeration limit, so with those computers you might be able to get away with connecting an unintelligent LED load and draw 500 ma., but some computers do enforce the 100 ma limit.
Thirdly, I don't know why you need 500 ma, or 12 volts for that matter. Most single LEDs need only about 2 volts to light up, and they draw only about 10 ma. to 30 ma. of current. So you should have no trouble getting some small LED or array of LEDs to light up with the 5 volts and 100 ma. guaranteed from the USB. Now maybe your "12 v LED" is really an array of 8 LEDs in series, which might very well required 12 volts. And maybe it has several of these arrays in parallel so that you want 500 ma. of current. But that would be a very very bright LED array! What are you really trying to do?