I have Music Maker Premium, but it seems I am unable to import MP3 content. It gives me a pop up saying I should download "More Tracks and Formats" from your store. Yet I dont see anything like that in your store. I have downloaded & installed everything I was able to.
In the store select More features option from the dropdown menu as shown below
The More Tracks and Formats should then appear
If presume you are using the Free version of Music Maker, if you want to do more advanced effects, etc then long term you would be better off buying the Plus or Premium version.
The free version is quite limited in what it can do with respect to the number of soundpools, instruments, effects and no way of adding additional VST instruments/effects available - see the comparison chart here.
Buying them as required can work out costing more than the Plus or Premium version.
. . . . . I purchased the Premium version as mentioned in my first post . . . . .
The message would then suggest that you have not downloaded and installed some or all of the extras that you get with the Premium version - these you will find in the store and they will have a download icon on them.