Every time you finish the work on the video and close the app, you are asked whether to save a project or not:

If you saved the project, you can open it after activating the program, and export it with no watermark (open via Recent project):

The project file stores all the changes made with videos, it also allows you to continue working on a video or export it as a video file. 

Please note that you do not need to re-add the video exported in the trial version, but a MEPX project file. This is the file that contains your work. 

Please see the menu File - Recent Project. Choose the one at the very top of the list as the earliest created.

Else, please look up the earliest created MEPX file on your hard drive.

If a watermarked file is added to a project, it will not export without it.

Please note that it is impossible to get rid of the watermark from the existing video, for example, recorded with trial version of Screen Recorder, or converted with Video Converter Premium. But if you have a *.MEPX (or *.MEPB, *.MEPS, or *.MEP)project file, you can re-save it after activating the program and then your video will be saved without a watermark.