Your problem with MF 7 SE arises from a little misconception on your part. When you have finished editing in VSX3, and save your work into the VSP file, you are not creating a 'film' as you call it, but merely a project file. A VSP file in fact contains no video (or audio). It is a small text file which merely tells the program which video clips are included in the project, where they are stored and what has been done to them. Unfortunately, MF7 SE cannot recognised project files (VSPs) from VS X3 -- it was written before X3 was even on the drawing board so the code cannot recognise X3 VSP files.
So what you have to do is to create a 'film' or a final actual video file of your edited project in X3. If you are wanting to produce a standard definition DVD in MF 7 SE, then you should, in X3, go to Share > Create Video File > DVD. This will create a DVD-compatible mpeg-2 of your project. (Note that if your project is longer than one hour, then you should choose Share > Create Video File > Custom, and reduce the bitrate so the final video will fit on a DVD. Roughly speaking, a bitrate of 8000 kbps will allow one hour of video to be burned to a single layer DVD; 6000 kbps will allow 90 minutes and 4000 kbps will allow 2 hours. Note further, however, that lower bitrates produce lower quality videos.)
Then when you have produced that new mpeg-2, you go to Share > Create Disc > MF 7 SE, and when MF 7 opens, you insert that new mpeg-2, build your menu and burn the disc.