Jailbreaking iPod touch with 3.0. problem with ipsw file?

i have the firmware for the 3.0 firmware and i need to put it in redsn0w. igot the update the right way from itunes and when i put it on desktop it is saved as .dmg. a video i saw said "just change it to ipsw" when i do that it warns me that it might not work perfectly or ne usable if i change it. firguring that the video i saw knew what it was talking about i clicked continue. in the redsn0w program, it says it cant recegnize the ispw file. what do i do? i just want to jailbreak it!!!