After downloading, open the disk image and drag the application to your hard drive. That's the installation process.
On launching MyMovieEditor, an Open... dialog will appear. Though all files should appear in this dialog, not all files are able to be opened by MyMovieEditor. After selecting a file and clicking on the Open button, the movie should appear in a window like this:
The (open) drawer on the bottom shows the times of:
Current Play Position
The movie responds as one expects for most things: Space bar for start/stop playing; drag the indicator to advance or rewind; 'i' and 'o' keys set the start and end of the selection, respectively.
But some keyboard navigation is different. To advance or rewind the left and right arrow keys are used. Withour modifiers, the movement is 1 second. Holding the shift key while pressing the L/R arrow key moves the slider by 1/10th of a second. See the below for complete combinations.
Typically, I open a movie, select the main part I want, use command-J to make a new movie from just the selection, and then Save... the new movie. When Saving a new movie, you have the option to 'flatten' it, or make it self-contained. Otherwise, it is saved as a 'reference' movie. To be useful, the reference movie will require the orignal movie to be available.
MyMovieEditor also allows cutting and pasting, though not to an empty new movie. One should be created from a selection in another movie first.