To enable HD you need to change the mode of your xbox to NTSC using Enigmah or some other such EEPROM utility.
Remember that if you are using NDure as your softmod then it will have a virtual EEPROM and so you will need to (temporarily) disable this before changing. (Assuming you want the change to be permanent).
After you have changed your box to NTSC you will be able to boot to the M$ dashboard from Unleash-X and change your setting to HD.
Here are a few details :