Well, this project kind of haven't been very active lately... But we'd love to have another driver... So the easiest way is probably to add you as a project member giving you SVN access (already done)... I doubt anybody mind that... :)
By the way, in SVN trunk there's a file called structure.txt and README... The readme gives a short overview, while the structure.txt defines how drivers, app, libs, etc should be added...
E.g. by adding a subdirectory to drivers and implementing the network.h interface if possible... Take a look at the enc28j60 driver if you want an example of how the driver specific makefile could look...
Note: At this point the biggest difference between this port and the official uip is probably a few avr specific configurations and the build system... Which I hope isn't too complex...
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Regards Jonas Finnemann Jensen.