how do I compile olsrd2?
olsrd2 is part of the OLSR.org Network Framework.
how can I reach the developers?
You should go right to the Mailinglists.
I want to write my own OONF Plugin
You should have a look at the Example Plugin, its a very simple OONF Plugin with all the basics.
There is also a document about the OONF Plugin Lifecycle.
I want to change the core code of OONF, not a plugin
Maybe you should get familiar with the code first, nearly all functionality that was core in olsrd moved to a plugin in OONF. The core is just made of three relatively small libraries.