Edit: Starting with driver version 0.6.0001, disabling Driver Signature Enforcement is not required anymore.
Note: 64-bit users: In order to run MotioninJoy on 64-bit, you must disable Driver Signature Enforcement
as MotioninJoy does not have a driver signature.
There are a couple ways of doing this.
First to check if you are running 64-bit: Go to Control Panel->System, and look under "System type" to see if you are running 64-bit or 32-bit.
(32-bit users and XP users don't need to worry about disabling driver signature enforcement.)
The most common way to disable the Driver Signature enforcement is after the BIOS boot screen press F8 (you may have to press it a few times) and select “Disable Driver Signature Enforcement”.
Unfortunately you have to do this at every boot to use the MiJ driver.
The other way to disable Driver Signature Enforcement is to 'sign' the drivers yourself. Download the Driver Signature Enforcement Overrider
app, then run it. You may need to turn UAC off for this work. You will need to sign all the files in the “C:\Program Files\MotioninJoy\ds3\drivers\x64” directory, most notably MijXfilt.sys and xusb21.sys. When you reboot your computer, you can now use the MiJ drivers without having to press F8 at every boot. The downside to this is you must have Test Mode enabled, and it will leave a little watermark at the bottom right corner of you desktop saying so.
Also check out the 64bit page
in the wiki for more ways of disabling Driver Signature Enforcement.