If you want to trim a few seconds from any video clip you have to use Videowave. Perhaps it is time you looked at some of the many tutorials on the web that show you how to use Videowave. Even the Help file for VideoWave (which is not horrible) gives a good description on how to use the Trim feature
Simply clicking on the Prepare for DVD Authouring will of course do nothing if you don't have a video clip on the timeline. Videowave has to be used if you want to do any editing such as trimming, splitting, adding transitions, effects etc.