You posted in 'Red Box' forums and it sounds like you have the 'Red Box' version (Easy CD and DVD Burning) so we will move in that direction...
The Red Box is a cut down version of the Creator Suites. Of the cuts is a full version of MyDVD!
In the Red Box the version of MyDVD will only accept Standard Definition video files and no MP4 files. Look at the list above the highlighted line for file types but keep in mind they must also be SD quality.
The Topic which you posted the Link to is based on a full Creator Suite and is aimed users who want to add Data Folders to a DVD Movie project disc.Your post does not indicate that this is what you want to do.
If I guessed right about all of this, you face rethinking... If you need to do HD or do not want to convert your HD clips or MP4 clips down to an acceptable format you need to return the product and get one that will!
One of the best replacements is the older Roxio Easy Creator 2012 Pro, pay close attention to the name if you are shopping on Ebay!
Roxio NXT 3 Pro is the second runner up Also out of production and while not certified to run under Win 10, both seem to work without any issues. These 2 will not only accept MP4 and HD source files, they both have a full fledged Editor as well as a full power version of MyDVD! (the more recent NXT 4 & 5 have a very poor version of MyDVD )
On the other hand, If you don't mind converting your source files to SD and accepted formats, Any Video Converter if free and does a nice job. And your output will be limited to a DVD Movie, then stick with the Red Box
More questions or need clarification, just ask!