You've probably thought of this, so forgive me if I'm belaboring the obvious. It may be a matter of how you have your boot order set. If you don't have it set so that the BIOS looks at your CD/DVD drive first, it will look at the HDD first and boot from that; it won't have any reason even to try to boot from the CD. The fact that you had this problem with two CDs suggests that this might be the case.
If this is the case, you can do one of two things. You can go into BIOS setup and change your boot order. If you don't want to futz around with that, you can simply force your computer to show you the boot menu by hitting the appropriate key as soon as it begins to start up. They keys are different with different computers and BIOSes. For example, on my laptop, if I hit Esc at power-up, it takes me to the boot menu, but on my other comps, the keys I have to use to do this are F8 and F12. If you have your manual, it might tell you which one to use. If not, look at the screen closely at power-up and see if you see a message telling you which key to hit for the boot menu. Often they'll mention two keys, one to take you to Setup (which you don't need unless you want to change the boot order permanently) and another to take you to the boot menu. If you don't want to make a permanent change, choose the latter.
Anyway, once you get to the boot menu, you'll see a list of available devices to boot from. Move down until the CD/DVD drive is highlighted, then press Enter. If your Linux disks are good, the CD boot process will start.
Sorry if I'm telling you stuff you already know, but HTH anyway.
Olivia 15 Cinnamon 64, dual-boot Win 7 Pro 64 SP1, Clevo W150ER 15.6" laptop, i7 3630M, 16 GB RAM, Samsung SSD 840 Pro 256 GB/HGST 1 TB 7200 RPM, Nvidia GT 650M 1 GB, USB 3.0 HGST 1 TB 7200 RPM, HP Photosmart C5280