This problem isn’t specifically related to MindView, rather it is a more general problem with the Default Programs set on your computer.
To resolve this issue you will need to take the following steps:
Step 1: Go to your Start Menu > Default Programs
Step 2: Select the option “Associate a file type or protocol with a program”.
Step 3: Select the extension .mvdx and see which program currently opens files with this extension.
To change the program, select the extension and hit “Change”, then navigate to MindView and select this as the program to open the extension.