If you click the mouse above or below the vertical extent of the staff line MusEdit will assume you are trying to position the caret in the line above or below the staff line.  This complicates things if you are trying to enter a very high or low note outside of the range of the vertical extent.  What you have to do is click the mouse somewhere within the vertical extent, then use the Up/Down Arrow keys to move the caret out of the vertical extent.  Since this motion is starting from within the staff line MusEdit will assume you are still in the staff line and will keep supplying ledger lines as needed (up to the maximum shown above).  Once you have the caret at the proper staff location, use the keyboard to enter a note with the keyboard code ( q=quarter note, t=eighth note, etc.)
 
Notes:

	If you need space between the line above and the treble line (to keep the high notes from getting tangled in the line above) put the caret in the line above, then hit Ctrl+T to insert a blank line, which will act as a space
	You will probably need to use Refresh (Ctrl+R) more than usual when dealing with notes outside of the vertical extent of a staff line (see Refreshing - Why Is It Necessary? for details)
	MusEdit doesn't automatically translate to octave ( 8va ) notation for notes which are unnaturally high above the staff.  If you want to draw very high pitched notes with 8va notation you will have to lower them yourself by an octave, then insert two "extended objects" above them: a text object (Ctrl+Shift+T) with the characters "8va"; then a phrase line (Ctrl+Shift+H) of the "Dashed Right End" type right next to it, as shown here: