Canon 1D Mark IV / 580 EXII - High Speed Sync Help Needed.

We've been invaded by hummingbirds and I'd really like to try my hand at capturing some with their wings frozen in motion. I can't seem to get high speed sync set up. I turn it on (on the 580 EXII) and the camera seems to stay at 1/300. I can't get it to go above that. Obviously I'm soing something wrong and I've really never messes with the high speed sync before. Can anyone give me a step by step on how I get this set up?

Thanks in advance!

I wouldn't even try with FP. Try to shoot with 1/300, but set camera so, that main light source is flash (read: without flash, photo is more or less black). Flash is most likely a lot faster then times you will get otherwise... unless you can get 1/80000 but then, there's no need for flash anyway. This way, flash will freeze movement and you are done.
Otherwise FP works with ETTL only. Set flash to ETTL, press button for FP mode, and here you go. At least with 580EX, if you press it twice it goes to second curtain... it cycles from normal operation, through FP mode, second curtain, back to normal operation, so check how many times did you press button

Have you tried the flash setting done via the Flash Function Settings on the camera menu screen, where Shutter Synch is set to Hi-Speed (not merely the flash's own menu screen)?