The rhythmic value of melody notes is set using the Play mode.  Just like there is a fast way to switch to Chords mode from Melody mode, there is a fast way to switch from Melody mode the Play mode:

the button displays the rhythmic value of the current melody note (assuming that note was added to the chords table).  When clicked upon, the program switches to Play mode ready to change that rhythmic value to a different one.

This is used occasionally because the normal way to setting the rhythm of a song is to set the rhythmic value of all melody notes in the Play mode.  Not  by switching back and forth between the Melody mode and the Play mode.

See The Best Way to Enter Full Songs.

When first entering melody notes, the default rhythmic value is assigned to the note that of a Quarter note.