When you put them in your tray, you need to restart your PC and select BIOS settings, the key varies from computer to computer, so look that up. When you go there, look for boot options or boot order, and make CD Drive first of them all, then save and restart. A message saying "Press any key to continue ..." or something like that, do what it says.
on my pc its f12 at start up and select the device to boot from
or press f2 and select the boot tab and change cd/dvd drive to top then save and restart