Try rebooting (take out battery, put back in). Hold down volume + and volume- (just hold the middle of the volume rocker), & then hold down the power button. Continue to hold the buttons down until the samsung flashes a second time, let go at this point. You should then go to a blue screen that says recovery, go to apply update.zip (this is cwm), use volume keys to go up and down, use the home key to select. You should then go into clockwork. Use volume key to go up and down once in clockwork, and the power button to select.
Once in clockwork you can flash a superclean rom, the newest is 2.8.
As far as mac goes, I really have no idea. I thought I remember reading a tutorial somewhere though.
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