You have to bypass the virtual eeprom before making the changes, otherwise they won't stick.
First, plug in your component cable, and install Enigmah to your E drive - E:\Apps\Enigmah\ is probably the most sensible. Next, bypass the virtual eeprom - how you do that depends on which softmod you have. For Krayzie NDURE you run Krayzie Extras and go into professional mode, for Kingroach you run E:\ndts\default.xbe and select system - file manager. If you aren't sure which softmod you've got, boot with Frosty's Rescue disc - that also bypasses the virtual eeprom. In all cases unless you have changed the default settings the LED will be red when the virtual eeprom is bypassed.
Once the virtual eeprom is bypassed and the LED is red, use file manager to run Enigmah from your E: partition to set the Xbox to NTSC. Reboot, then bypass the virtual eeprom again, and run the MS dash (C:\xb0xdash.xbe) to enable the HD modes.
Finally, if you're running Krayzie, run Extras again, and select "Update virtual eeprom".
Reboot, and your Xbox will be in NTSC with the HD modes enabled.