Press F8 while booting up, as soon as the ASUS logo appears. You have to do it very early in the boot sequence, so it may take you a few tries. Sometimes pressing it repeatedly as it boots works best. This should take you to the boot menu.
From there, choose your DVD drive or your USB drive. USB (flash) drive is likely to be under the list of hard drives or even SATA devices.
If you choose DVD drive and it spins the DVD drive, but still does not boot from it, then you may need to press Enter to confirm starting from DVD; if that's the case it will prompt you. If it spins the DVD (lights come on and drive makes noise), but still fails to boot, then there's probably something wrong with the DVD.
If both of those fail, try turning "Legacy USB support" either On or Off on the Advanced tab in the BIOS.
Trial and error is your friend in this case. Try every possible combination and one will work, assuming your DVD and/or USB drive are truly bootable. If you've got another system, try booting from the DVD and USB drive on that system to verify that they are indeed bootable.