chances are its probably had a leak with the previous owner and is now sealed shut so opening it can possibly cause you problems when the rain returns.
if you want to get the glass out you should use the red button and then the lever should release for you.
if you want to just open the sunroof then use the green button but again if the mechanism is working (which it looks like it is) then the sunroof is possibly a little stuck through lack of use or more likely been sealed down to stop leaks.