In the Mixer window (Window > Mixer), select 'New...' from the File menu field.
A new destination will be displayed in the Mixer. You can give the destination a name, port, channel and color. Choose 'Internal Synth' as port, and '1' as channel.
In the Parts window, right click on "Untitled Part 1" and select 'Rename' from the context menu. Call it "Piano" and hit return.
Drag & drop the part into the Assembly window, where a blue-box style representation of the part will show up.
Back in the Parts window, double click on the "Piano" part and a window for editing that part will come up.
Use the pencil tool to draw a few notes in the piano roll strip. You should hear the notes being played back when creating or modifying them.
Press the play button in the Transport window. You should now be hearing the notes you entered before.
You can adjust the velocity of the notes (which is often directly mapped to their loudness) by clicking in the 'Velocity' strip and dragging the mouse cursor over the velocity curves you want to change. If you use the left mouse-button, attack velocity will be affected, while the right mouse button changes release velocity.
To change the sound of the notes, you have to insert MIDI program changes. Select the 'Program Change' item from the pencil tool drop-down list and click into the 'Sequence' strip. A new event with a horn icon and text will appear, resembling the program change event. The left offset of this event determines when it will take effect. In the Inspector window you can change the program by dragging the corresponding slider.
If you'd like to speed up or slow down the song, you have to adjust the tempo control in the Transport window. Do this by either dragging the digits vertically, or by right-clicking the control and manually enter the speed (which is measured in bpm, beats-per-minute).

Some other random notes:

Cut, Copy & Paste is not fully implemented. Within windows/strips you can copy events by dragging them while holding down they control key, which will effectively clone the selection.
You can use the fancy 'lasso-select' mode to select multiple events that otherwise couldn't be selected just using rectangle mode. Start lasso selection by right clicking in the strip and dragging the mouse around the events you want to be included in the selection.
Make use of the 'Repeat' event to loop sections of the song.
When you load a song that uses the BeOS Internal Synth, MeV might need some time to start up the synth, and this currently looks a bit as if the program had crashed for a split second. Better indication of what's going on will be added some time in the future.

In addition, be sure to check out the included example songs. They're as cheap and cheesy as you'd expect from demo songs... well, I guess they're even cheesier. But they might help you understand the concepts and interface of MeV.