With MEP closed, use Windows Explorer to search for the file videodeluxe.ini (MEP17) or movieedit.ini (MEP16 and earlier)and delete it then restart MEP you should then be able to adjust the media pool window as you need. Note you will have to reset any custom filepaths you have created.
The location of videodeluxe.ini depends on your version of Windows and how you installed MEP, you may need to set your Folder and Search options to show hidden and system files. Donot forget to reset these settings when you have finished.
Please include in your posts your MEP and Windows version and if necessary your computer specification.
Which version of Windows are you running - the reset not fixing the problem suggests that the .ini is not being written to with the new settings when MEP is closed - which in turn could be a permissions problem.
Which install did you do - All Users or Single User can use MEP? If Single user try uninstalling and reinstal so All Users can use MEP.