What is happening when you try to boot? Does your system see the DVD drive and ask if you want to boot to it? Are you selecting it in the boot devices menu?
I have an ASUS Z77 board, but not an external DVD. I just use a Flash drive, if I don't want to use the internal DVD.
What type of connection does the DVD have? Do not use a USB 3.0 port to boot it, but it should see it during the initial install even if you do.
Are you wanting to install using the UEFI or Legacy bios?