You need to select an instrument in order to hear a sound with the keyboard. See your image, select Software instruments and select and instrument. Alternatively, click in the box at the very left of track one with the + on it and it will open the list. If there are none, it is possible that no instruments are loaded with the trial version.
Normally when the software is installed and registered, you have to go into Help, Update program/Update functions, Download free synthesizers and sound packages, in order to get all of the instruments.
A couple of things, if you buy MMM, get the Premium version. One important point, it is not a DAW, close, but not quite there. You can do a lot with it, but you can only record to one track at a time. Samplitude Music Studio allows recording to more than one track at a time.
If you want to see music that you make in standard music notation, you cannot; piano roll only.
If you want to score or dub videos, then this is the tool. You can import the video and see it in MMM. In Samplitude Music Studio you cannot. That is why I have both.
Thanks again for your response. Yes, I'll definitely buy MMM the premium version since it contains so many interesting features in it. Actually the reason why I want to buy MMM is because I want to make my own music, hopefully to sell it:-)
I'm actually a beginner & know very little about MMM but trying to learn & make the best of it.