This can be due to the DVD region format. DVDs come in two formats - NTSC and PAL. miDVD sets this setting automatically, but sometimes you need the other format.
To get around this, go to Preferences and set DVD Type to Always NTSC and burn the DVD again. If this does work, burn another DVD but this time with a DVD Type of PAL. In the free version of miDVD, DVD Type can be accessed from the File Menu.
If this does not help, send us an email and describe exactly what you see when you try to play the DVD.