Apple - How can I disable kext signing in Mac OS X 10.10 Yosemite?

To disable the kext signing security setting:

sudo nvram boot-args=kext-dev-mode=1

After changing this setting you need to restart the computer to have OS X recognize it.

It is important to note that the kext-signing setting is global, if you disable it you should be careful to only install system drivers from sources that you trust.

To reenable again use:

sudo nvram -d boot-args

Quoting Apple's documentation:

To enable or disable System Integrity Protection, you must boot to Recovery OS and run the csrutil(1) command from the Terminal.

Boot to Recovery OS by restarting your machine and holding down the Command and R keys at startup. Launch Terminal from the Utilities menu. Enter the following command: $ csrutil enable

After enabling or disabling System Integrity Protection on a machine, a reboot is required.

Source: Configuring System Integrity Protection

Still quoting:

As of OS X El Capitan, the kext-dev-mode boot-arg is now obsolete.

sudo nvram boot-args="kext-dev-mode=1" # Has No Effect

Source: Kernel Extensions