A couple things to try:
1) If you're using UEFI boot (vs Legacy boot), you may need to manually add an entry to the boot menu, and hit F12 on startup to boot from DVD. Check what kind of boot method is being used and see below:
How to enable boot from DVD option with UEFI boot mode enabled. (Windows 8) | Dell US
2) Another possibility is that you have the wrong SATA PORT TYPE set.
- Check your "SATA TYPE" setting, it's probably set to AHCI.
- Then see the "SATA PORT TYPE" for where the DVD drive is (usually Port 4/5). See if you need to toggle between the settings (usually either "As SATA" or "IDE").
Edit: just saw that you're using Windows 7. Then you may be in Legacy boot mode and #2 is more likely to be a factor.