On a typical Dell, U can simply go to F12 menu (tap F12 key many times at the Dell logo screen), it's gonna take to boot device menu. Choose the one option that mentions ur CD/DVD ROM (it might even say USB CD-DVD ROM drive), but that's ok.
U may wanna also check HDD is recognized by BIOS. if not u better look at the HDD connection to power & data, & from HDD to MB (the same data cable).
If BIOS doesn't recognize it & connections are good, try a different data port on the MB.