There are a few methods depending on how "stuck" it is:
Method 1 :
Use Disk Utility to eject The first
and simplest method, if you only
want to unmount a single volume on
the disk, is to use Disk Utility,
located in Applications/Utilities.
In Disk Utility, simply select the
volume you want to unmount and click
Eject.
Method 2 :
Hold Down the mouse button at
startup In some cases holding down a
connected mouse button at startup
will cause a misbehaving optical
drive to eject its media.
Method 3:
Boot into Open Firmware and eject If
you have a Mac that will not startup
properly and has a stuck disc, try
booting into Open Firmware by
holding down Command, Option, O key
and F key during startup. After
booting into Open Firmware, type the
command eject-cd.
Method 4:
Using the Terminal. There are two commands that can be used in the Terminal (located in Applications/Utilities) which can be used to force disk ejection:
drutil tray eject.
Simply type in the above command and press return.
Last resort method (Danger, Will Robinson!):
Also knwon as, Ye olde screwdriver method.
- Turn the CPU upside down and lay it
flat on a desk or table. Use a
flat-edged knife or small pry bar to
gently and carefully lift the bottom
panel off the unit.
- Use a small Phillips-head
screwdriver to remove the four
screws at the corners of the large,
flat, shiny metal panel directly
below the bottom panel. Lift off the
panel to expose the CD drive and
remove the CD carefully with your
fingers.
- Replace the metal panel and its
screws, then snap the bottom cover
back into place. Reattach all of the
cables and boot up the computer.
If you still can't it out using the last method, then you a really, ahem, stuck.
Call the fruit company.
;-)
On a PPC Mac such as your G4 Mac Mini, you have to use a Firewire drive. USB booting wasn't really supported before the Intel Macs. You can use a Firewire optical external drive, a Firewire harddrive that has the OS Installer restored to it, put the Mac mini into Target Disk mode and install OS X on a Mac with a working optical drive, or you could even use NetInstall on a MacOS X Server machine.
Best Answer
Simpliest thing to do: on a 64 bit pc with cd drive put the openbsd install cd and plug the usb flash drive, boot and install on the flash drive as a regular disk. When finished, copy ftp.openbsd.org/pub/OpenBSD/5.2/amd64/*tgz in the directory /5.2/amd/ on the flash drive. Then plug this flash drive in the mac mini and at the boot prompt type "boot hd0a:/bsd.rd", proceed as usual and when prompted for sets choose "/5.2/amd". That should work (I don't have any mac), at least it works on any pc/netbook/notebook, let me know plz ;)