The MOUSECLICK() output depends on how many buttons you have on your mouse. I just use a standard two button mouse with a mouse wheel so I only wrote code for that. But the general rule is each button has a number assigned to it and if you want two or more buttons to be pressed at the same time you add each button number.
` Set text to overwrite black areas
set text opaque
do
` Show instructions and current mouseclick() status
text 0,0,"PRESS A MOUSE BUTTON"
text 0,30,"MouseClick() = "+str$(mouseclick())
` Clear text
for t=60 to 160
text 0,t," "
next t
if mouseclick()=0 then text 0,60,"NO BUTTONS PRESSED"
if mouseclick()=1 then text 0,80,"LEFT MOUSE BUTTON"
if mouseclick()=2 then text 0,100,"RIGHT MOUSE BUTTON"
if mouseclick()=3 then text 0,120,"LEFT AND RIGHT MOUSE BUTTONS"
if mouseclick()=4 then text 0,140,"MOUSE WHEEL"
if mouseclick()=7 then text 0,160,"LEFT, RIGHT, AND MOUSE WHEEL"
loop