First, make sure your US428 control port is selected as an input and output in Options – Preferences – Midi/Sync.
Then go to Options – Midi Triggers. Click the button on the bottom – Enable Midi Input Sync. Select the function you wish to program, and press the button on your US428 you wish to setup as the control, and it will program it.
Here’s a tip though- the US428 sends two messages- one when you press the button and one when you release it, and Sound Forge will pickup the last message from the 428 which will be the release message of that button. If you want fast smooth access to the buttons when you actually press them, press and hold say the play button, then on screen, click on another controller you don’t wish to use, and release the 428’s button. That way the button press message will appear on the play, and the release of the play button will appear on another one which doesn’t matter. You can use this dummy control over and over.
You can also program the knobs (but faders will be weird to program) because they simply send a message if you turn them clockwise, and another if you turn them counterclockwise. Save the configuration when you’re done, and that’s all there is to it.