Cubasis project importer compatibility is given with Cubase Pro 8, Cubase Artist 8, Cubase Elements/LE/AI 8.
Although not being officially supported there is a small chance it might work with older Cubase versions also.
Here is a great Music App Blog tutorial that explains the workflow:
Please make sure to have the latest available Cubasis project importer version installed (http://www.steinberg.net/index.php?id=content_and_accessories_cubasis) and give it a try.
Hope that helps.