The major problem here is that SolPart is not a template. Thus, you cannot specify it as a menu style.
If you have the site currently running and it uses the old SolPart Menu, then review the DDR User Guide and make the appropriate changes to the existing skin. You'll probably want to use "dnn" as the tag rather than "ddr," too.
If this is a new skin, then maybe you want to start with the Superfish menu style. You'll need to download the template from Codeplex, as described in the User Guide. I've found it fairly easy to adjust the Superfish menu's CSS file to meet a variety of needs.
If you do a search for "ddr" on this site, you'll find quite a few tutorials, including on on styling the Superfish menu. Use this URL as a shortcut to the search:
http://www.dnncreative.com/search/ddr