First of all, setup the AJAX Remote as described in the FAQ entry "How do I setup the AJAX Remote?".
Then follow these steps:
- In AJAX Remote on your iPod Touch or iPhone, tap on the "Music" symbol.
- Now tap on "Library" (upper-right corner) and navigate to the media file you want to playback on your iPod Touch or iPhone.
- Tap on the blue play button.
- If QuickTime thinks that it can playback the file, you can now start playback by tapping on dark-blue playback button.
All preference settings are already set correctly by default. If you don't see any playback symbols: the remote access to files is only supported if you have set a password ("Require password" is enabled) and the option "Allow access to the files in your iTunes library ("streaming")" is enabled as well. Both options can be found at Preferences > AJAX Remote.