Simple custom widgets can be added using the Palette Manager
Custom viewers typically don't conform to any specific API, so you need to use the WindowBuilder Customization API
to add it to the palette.
See the New Component Tutorial
and Customization API
See the following *.wbp-component.xml file for this viewer. Put it in the same package as the viewer class, or into a parrallel wbp-meta folder with the same package structure.
You will also need the following palette file to contribute this viewer to the existing JFace category. It should go into the wbp-meta folder.