Select "Disk Utility" from the "Installer" menu when you boot from the OS X Install CD/DVD, then select the "Erase" tab, then click "Security Options..." There you will find the 35-pass erase option.
Be aware, though, that the 35-pass operation will take forever to complete, and I'm not talking a few hours. Perhaps an entire 8-hour day or more. If you're a secret agent, and must, under no circumstances, allow anyone with the most advanced technology in the world to recover your data off of your hard drive, then, by all means, use the 35-pass erase option. If you're just looking to erase and install a fresh and clean copy of Mac OS X, then simply use the "Don't Erase Data" option in that security pane.
The 35-pass option is for making absolutely sure that no one, anywhere, with any amount of technology, can open your Mac mini, remove the drive, remove the platters from the drive itself, and examine and reconstruct data off of them. It's strictly for the most advanced security, not for correcting errors on the drive or making the drive perform any better.
