To import the synthclipse-demos project to your workspace do the following steps:
Press the Finish button and the new synthclipse-demos project should appear in the Project Explorer View. If you open some GLSL file and there will be errors in the editor, you might need to rebuid file index of the project to get rid of them.
If you have already imported synthclipse-demos project and just want to check whether some new demos were added press a RMB on the project and choose Team -> Pull. This will update your project to the latest version from the repository.