Make sure MAC is filled in on the Media Director that you added as your diskless boot "Generic PC as MD. When entering MAC address enter in the following format 00:12:24:93:AC:E4 with colons seperating the numbers.
You should a similar number on the pc that is trying to PXE boot, hit pause if needed to see mac address. Just enter your address where it says MAC, Select update and then select "Set Up Diskless Media Directors" Quick Reload Router" I like to reboot. Reboot your PXE PC. Once this done the PXE boot pc should find an image and go through about a ten minute process to set everythying up.
Hope this helps