Hey Ruud,
Well, there are a few ways to do this. I'm going to make the assumption that you don't want your users to be able to freely download your trials and use them right away (which you could do by setting the trial length and using the UseTrial() and GetTrialDaysRemaining() function). I'm assuming, rather, that you want the user to request a key from you so they can begin their trial. Here's how you can do it based on these assumptions:
Set your trial length to 0 (like you did) and generate an "online" trial extension key for however many days you want the user to use the trial. Then you just need to add a simple form to your app where they can enter their trial extension. Just make a call to TurboActivate.ExtendTrial() to apply the trial extension.
Also, you'll need to make use of the UseTrial() and GetTrialDaysRemaining() functions.
As far as the registered produce protection goes, you can simply follow the included example project.
Tell me if this helps.