How do I get full resolution on external monitor with MacBook Pro

Hi,

I have an early 2011 MacBook Pro:

https://support.apple.com/kb/sp620?locale=en_QA

And it states I can run an external monitor at 2650x1600. I have bought a BenQ BL2710 which goes to 2650x1440:

http://www.benq.com/product/monitor/bl2710pt/

I can only get 1080p - the OS (Mavericks) does not have any option to go higher. I have tried connecting MiniDisplayPort to DVI via an adaptor, and I have tried MDP to HDMI using an adapter. Have I missed something?

Thanks,

Nick