Direct Disk Boot problem

Sorry for my newbie question, I'm trying to install FreePBX13 (based on CentOS) on linode. I was following the instruction on 'Install a Custom Distribution on Linode' but i got stock as the RAMDISK was not large enough to download the image via wget, so i downloaded to sda. When trying to boot from disk i get a failure indicating there is no MBR.

And i don't know how should i create a MBR for the disk, can anyone help with a doc or hint. :?