Update(1/17/13): Leven w520 and LinuxMint 14 combination makes it very difficult use the external monitor. After a few trials I gave up on it. I tried so hard and disappointed, I gave up on w520 and got a MacBook pro.
However Lenevo w510 and LinuxMint 11 combination worked very well for the external monitor. So w520 +linuxmint 14 are bad combo. I heard people have success with W530 though.
=== You mileage may vary with the following tricks ========
My Lenvo w520 came with Windows 7. When I installed LinuxMint on it, I could not get the display on an external monitor. Also clicking on NVidia X Server Settings in the Control Center window give a warning saying NVidia driver is not in use.
Found out the problem is the Display setting in the BIOS. W520 has the display card work in 3 modes, integrated, discrete and NVidia Optimus. NVidia Optimus is meant only for Windows 7 hence it was configured that way by default.
Switching the setting to Discrete mode make everything works as expected.