Converter, not Adapter
The problem with getting a DVI output from an old MacBook to the DisplayPort input of the new LED display is not one of pins, but of signals.
My understanding based on a bit of reading when DisplayPort was announced is as follows.
DisplayPort is a relatively new standard and puts a little more intelligence in the display itself. With DP, the computer tells the monitor "here's an image, go render it". This allows it to do smart things, like to only update regions of the display if they have changed
With DVI, the computer has to drive the display directly. Giving it a constant stream of data to render as it sweeps across and down the screen. Although DVI is digital (well, at least DVI-D is), the principle is still the same as the old VGA standard - it is based on the idea of a beam of electrons sweeping across a coating of phosphor like old CRT monitors.
[Sidenote: HDMI is just DVI with added audio]
Thus a DisplayPort cable is more akin to a data cable - say USB or FireWire - than to a video cable. Well, the cables are probably very similar - being twisted copper and all that - but the way the port works is more like USB or FireWire than like VGA or DVI.
DisplayPort is backwards compatible in one direction. The standard allows for a DisplayPort SOURCE signal to be output as a VGA or DVI signal - which is why you can get adapters that would allow you to connect a new DP MacBook to an old DVI monitor.
However, the Apple LED display is not compatible in the other direction. It does not have the ability to be driven by a VGA or DVI signal. It can only read and react to a DisplayPort signal.
So the problem you have is not about figuring which output pin to connect to which input pin. It is about how you transform one format of data into another format.
What you need is a converter, not an adapter.
Much like you can get breakout boxes that allow you to convert, say, analog audio from a guitar to digital audio via USB, or say, analog video from a VCR into digital video via FireWire, you will be able to get a box that converts from DVI to DP... but, it won't be pretty and it won't be cheap.
Effectively the converter will have to implement the image processing to convert the signal from DVI to DP. If analog-to-digital video converters are anything to go by, often these are shoddily implemented, limited in the kinds of signals they can process (e.g. the resolution and refresh rate of the input DVI signal) and cheaply assembled. Yet, they are expensive because they are in-demand specialist equipment and because they also have to license the DP standard.
