all I can do is tell you what I've learned here-try either one of these-hold down the shift key on startup/hold down the mouse on startup/if not, See if you can boot up in single user mode/reset NVRAM,(command option, o+F)on reboot . Failing that....pull down the optical drive door, and using a straightened extra study paperclip, find the small hole near the optical drive tray. Stick that paperclip in and the tray should open up just enough for you to grab it and put the cd in there. Maybe then, start up holding down the C key......
just a guess though
Good luck to you