Welcome to Sweet Home 3D development. [:)]

How can I contribute to the project?

If you're a Java developer, the best way to start contributing to the project is to create a plug-in, with the help of the plug-in developer's guide. Once ready, you can post it in the plug-ins tracker to share it with Sweet Home 3D users.
If you're looking for ideas of plug-ins, please explore this forum.

I can't open the eclipse project that is in the source code directory. Can i also use netbeans?

Sweet Home 3D project is developed with Eclipse and there's no reason which should prevent you to open it in Eclipse (don't forget to read the README.TXT file in the source code). You should be able to use the project in Netbeans too.

Do you use UMl for the design?

There are two class diagrams available in the plug-ins developer's guide, but they aren't up to date.

What kind of techniques do you use for the 3d view?

The 3D view uses Java 3D API.

Is there any prerequisite for joining the project?

Not for the development of plug-ins.

Is there a special developer website / envoirment

Follow the developer's guides link in the menu.