Some examples of usage:

In this first procedure, the turtle follows the mouse when it moves on the screen.

to example
# when the mouse moves, go to the next position
if readmouse=0 [setpos mousepos]
example
end

In this second procedure, it's the same but you must click with the left button of the mouse if you want the turtle to move.

to example2
if readmouse=1 [setpos mousepos]
example2
end

In this third example, we create two pink buttons. If we left-click on the left button, we draw a square with a side of 40. if we left-click on the right button, we draw a little circle. Last, if we right-click on the right button, it stops the program.

to button
#create a pink rectangular button (height 50 - width 100) 
repeat 2[fd 50 rt 90 fd 100 rt 90] 
rt 45 pu fd 10 pd setpc [255 153 153]
fill bk 10 lt 45 pd setpc 0
end

to lance
cs button pu setpos [150 0] pd button
pu setpos [30 20] pd label "Square
pu setpos [180 20] pd label "Circle
pu setpos [0 -100] pd
mouse
end

to mouse
# we put the value of readmouse in the variable ev
make "ev readmouse
# we put the first coordinate of the mouse in variable x
make "x item 1 mousepos
# we put the second coordinate of the mouse in variable y
make "y item 2 mousepos
# When we click on the left button
if :ev=1 & :x>0 & :x<100 & :y>0 & :y<50 [square]
# When we click on the right button
if  :x>150 & :x<250 & :y>0 & :y<50 [
          if :ev=1 [circle]
          if :ev=3 [stop]
]
mouse
end

to circle
repeat 90 [fd 1 lt 4] lt 90 pu  fd 40 rt 90 pd
end

to square
repeat 4 [fd 40 rt 90] rt 90 fd 40 lt 90
end