If you wish to develop extensions to the OpenSAML 2 library these instructions will help you set up your developement environment.
First, if you are using the Sun JVM you MUST endorse the latest Xerces and Xalan.
Next, check out the libraries from the repositories listed below. Instructions for doing this in Eclipse can be found here.
For Java Development: java-xmltooling, java-openws, java-opensaml2
For C++ Development: cpp-xmltooling, cpp-opensaml2
Maven is required for building the java projects. If using eclipse, the m2eclipse plugin should prove useful. After that, it's just Java development as normal.