As far as I know, the mx518 is a 8-buttons mouse? -If so, then you have to set
Option "Buttons" "10" #instead of "7" like in my example above
I'm not sure, but I guess that this will also cause to shift the wheel-buttons, so if you are not able to scroll anymore try:
Option "ZAxisMapping" "7 8"
Maybe it is also necessary to re-map some buttons, so that every button does what you want it to do. You can "try" different mappings with the command 'xmodmap'. So open a terminal and type something like:
xmodmap -e "pointer = 1 2 3 4 5 6 7 8 9 10"
Open a web browser (or something else that supports "many buttons") an check if everything works the way you want. If, in example, button 9 and 10 do what you think button 4 and 5 actually should do, then run again 'xmodmap' but this time:
xmodmap -e "pointer = 1 2 3 6 7 8 9 10 4 5"
Remember that 'wheel-up' as well as 'wheel-down' are also just buttons (so you can also configure that scolling up and down works like left- and right-button) Play around with these numbers until everything feels good for you, or ask google; I guess that someone already figured out the usual button-mapping for your mouse.
However, when you know the correct order then write it to the file '/etc/X11/Xmodmap' (maybe you have to create this). The file should contain something like:
pointer = 1 2 3 6 7 8 9 10 4 5