First, please read this post and give us all the information requested there. At the moment we know very little about your computer nor even the exact version of MEP Plus you have (you can check this via Help>About).
Then, please download and install the free MediaInfo utility, point it to the file that MEP fails to play and post the results of the Text view here.
The problem is almost certainly being caused by a "missing" codec and MediaInfo will show us which codec(s) was/were used to create the file and hence, which one(s) is/are needed to play it.