This is an email we get more than once a week. And 99% of the time it is one of two things.
1. Not having proper Nvidia Web Driver loaded and running
2. Not having power to needed pins on GPU
It is easy enough to check the web driver, it has a little icon at the top of your OS X desktop. Click on it and make sure it lists the Nvidia Web Driver and not the OS X default driver.
As far as #2 goes, there are different things that apply to different cards. If your card has dual 6 pin power inputs it should be pretty straightforward, plug them both in to the connectors on the Logic Board. (shown on our install page)
When there is a 6 pin and an 8 pin, things get more complicated. On many GTX680 cards the 8 pin is "optional". This means you can plug in a 6 pin and the card works fine. For many GTX770 and GTX780 cards we do a mod to the card that creates this same functionality. So you can just plug in the 2 @ 6 pin cables and it should boot right up.
For cards like the Titan And Titan Black we have historically not done this mod for the reason that there are cases where there will not be enough power from just 2 @ 6 pins. For these cards we recommend an external power supply. And here things get tricky again. To most people, a power supply that is plugged in and turned "on" would be ..."on". But they typically are NOT.
External power supplies take many forms. The Booster X5 and the VisionTek version of this both require a Molex plug that goes "live" to trigger them to turn on. In a Mac Pro 1,1 through 3,1 this is easy as the optical bay that these supplies fit in have those exact plugs sitting right there, and they do go live whenever the machine is on. For 4,1 and 5,1 Mac pros the plot thickens. To get a live Molex, you need to adapt one of the SATA power outputs to become a Molex that fits in the back of the PSU.
Here is an example:
You may need to break off one of the plastic tabs to plug this into the Mac Pro connector.
As far as using a regular PC power supply, most of these will require either a "paperclip trick" or the better choice, a SWEX. This gives you a nifty switch to flip. The easy way to tell when most PC PSUs are on is when their fan starts spinning.
If you decide to use a 6 to 8 adapter, be aware that some of them that use 2 @ 6pins to run into the 8 only one of the 6 pins coming in will give working output by itself. Oddly, it is the side with fewer wires that will work. I suggest only using such a thing for testing.
Let me know if anything here is unclear and I will try to make it easier to understand.