I'm new to Music Maker but I think you need a plugin for this - More Tracks and Formats. You can see this in the More Features dropdown which is accesed from the shop tab.
To add to what @RaveyDavey has written: mp3 export is available ONLY in the purchased versions of MM, not the Free version. You can "add" the facility for mp3 export either by selecting to buy only that feature but the recommendation here is to buy the full Premium version of the programme. It often turns out to be cheaper in the long-term than buying the separate extra facilities you want to use.