Cannot boot from a bootable CD through USB

I can't boot from a separate CD that is connected with USB to too laptops: DELL running WIN 8.1 and a Lenovo S400 running WIN 7. i tested the CD in an old XP machine with an internal CD and it worked.
I think once my DELL booted from an other bootable CD using USB but i am not 100% sure.
I set the BIOS on the Lenovo to enable boot from USB. there aren't many devices to choose from in the Lenovo BIOS, only 2: the hard drive and to boot from network.
Both computers booted from the internal hard drive.
How do i make a screen shot of the BIOS?
thanks

I uploaded the snapshot of the Lenovo S400 with WIN 7

Is the USB plugged in when you took that picture?

If not, plug it in, if it is, try a different port, USB 2 preferably.

Here's the info on your one time boot menu option,



http://www.lenovo.com/shop/americas/...s300_ug_en.pdf

It worked for the Lenovo but the Dell doesn't boot from USB CD. i attach both pictures, the BIOS and the boot menu.
There is a line in the BIOS setup: USB Storage Device. this line disappears in the boot menu, as you can see in the pictures.
What does the last line in the Boot menu mean: Boot List Option [Legacy]?
And what happens if i change the value from legacy to something else? i tried but was afraid because there was a severe warning.

Is the USB CD recognized as CD/DVD/CD-RW Drive or as a USB Storage Device?

The pictures here are of a Dell laptop, not a Lenovo. the problem with the Lenovo was solved, thanks to your help.
I don't have an internal CD in my laptop, so i connect a USB CD.