Open Steam, click on File and pick Change user.... Confirm and log-in into your account again. Might solve your problem. If it doesn't exit steam then go to your steam folder and delete ClientRegistry.blob. Run steam again and see what happens:)
I would go for the MX518. Sweet little piece of equipment. and you will need a mousepad if you want optimal gaming. I have a combo of MX518 and razer exactmat and they truly pawn!