Hello @Mikey P,
When a MixPad project is saved, a MixPad Project file and a Project Data folder are saved in the same location, the MixPad project file <name>.mpdp and the folder <name>.mpdp.ProjectData.
Both must always remain in the same location because when the MixPad Project file <name>.mpdp file is opened, it pulls the tracks from a folder in the same location with the same name plus ".ProjectData". For example, a MixPad Project file named "Ultra Sound.mpdp" will look for, in its same location, the project tracks in a folder called "Ultra Sound.mpdp.ProjectData". The MixPad project file will not remember the location of the Project Data if it is moved alone, it will always look for a folder within the same location, if needed, move them together.
As names are very important to be able to open MixPad Projects, do not change the name of the MixPad Project file or Project Data Folder, if you do, make sure the <name> portion is exactly the same for both.
Try putting the original MixPad Project file and Project Data Folder in the same location, then open the Project file, it should be able to open the whole project.
DO NOT try to open a Project Data Folder with a different Project file, even if you change the names to match as this might cause the loss of tracks or audio in the tracks.