You'll hear a fragment of melody invented by the computer - very brief; as the name of the activity suggests, it's just one measure. This melody is made of only two note values: quarter notes and eighth notes (crotchets and quavers if you're British). The tool palette will contain just those two notes - your task is to put them in the right order in the measure. You don't need to think about pitches, which will be automatic. Just think about the rhythm.
Since only rhythm counts here, your staff has just one line and the notes will all appear on that line. To enter a note, touch the desired tool (quarter note or eighth) and then touch in the staff area. A second touch will enter another of the same value, or you can touch the tools palette again to choose a different note value.
The quickest way to change your answer is to use the Undo button, the left facing arrow found on the right side of your screen (shown at left). Alternatively, you can delete selected notes by swiping left with one finger. Touch a note to select it, or drag a selection rectangle around a group of notes.
Use the Play button to listen to the example as many times as you wish. If you're having trouble, try turning on the metronome to hear the beat.
Before checking your answer you can use the "Hear your version" button to compare what you've written with the original. The melody will display in staff notation when you check your answer.
Once you are able to do this beginning step, you'll be ready to move on with learning to write melodies by ear - which of course is what composers do. You can try Rhythm Dictation as your next step, writing more complicated and lengthier rhythms by ear, again not worrying about pitches. Then you can do Pitch Dictation, in which you think only about the pitches. Finally you can try the full Melodic Dictation exercise, in which you are responsible for both the pitch and the rhythm. Master that and you'll be one of those people who can write something down when you hear it.