To open a MMM project in Samplitude Music Studio (SMS), simply open it or drag the MMM file onto the arranger and it will open.
What is wrong with the new version of SMS2015? It looks to me like everything is there, just not the way it was before. You now use the Record, Edit, Mix, Master and Export views to get the appropriate buttons. The interface is less cluttered. Opening an instrument or synth is much easier and clearer.