Using Virtual Media to Boot from CD using Avocent DSR2035 and PE6850

Has anyone managed to boot a dell server and install it by mounting the dell install cd as virtual media using KVM over IP from avocent. Trying to get it to work on a PE6850, only despite seeing the virtual devices in the boot menu, and seeing them during the startup, nothing happens. It looks like the ability to boot from USB flash drive is missing from the Hard-disk Drive Sequence in the bios (A04). Any ideas how to get it there? 

Avocent say i need to check the server is USB 2 composite compliant/bootable? Any ideas?