Bumblebee: Cannot access secondary GPU - error: [XORG] (EE) No devices detected

The following AskUbuntu answer fixed it for me: https://askubuntu.com/a/285638/47280

Basically, you need to edit /etc/bumblebee/xorg.conf.nvidia and uncomment the following line:

BusID "PCI:01:00:0"

In the config, it states that this is necessary for Ubuntu 13.04. This number refers to the BusID of your GPU, so if it's different then go to the other answer for full instructions.

I found the following file:

/usr/share/X11/xorg.conf.d/10-nvidia-current-latitude-e6530.conf

I deleted it and optirun worked!

In /etc/bumblebee/xorg.conf.nvidia , uncomment

BusID "PCI:01:00:0"

and replace with

BusID "PCI:0x:00.0"

where x is the number that you see in 'lspci' list next to 3D controller, which should be your nvidia card