This is known issue 111953:
Issue: If you use the Management Client with Webstart on a Mac OS X client, you might not be able to create new elements or edit existing ones. When you attempt to create or edit an element, you might see a Class Cast exception error dialog box.
Workaround: Add the following line to the SgClientConfiguration.txt file in the .stonegate folder of your client:
This fixes the issue and also improves the rendering in Mac OS X
PS. We had one case where this didn't help at first as SGClientConfiguration.txt file was old and was missing couple of other parameters. So if adding above parameter doesn't help, delete the current SGClientConfiguration.txt file from your Mac, connect to SMC webstart page and download management client again. This will create new SGClientConfiguration.txt file on your Mac and then adding the above parameter will help.
Thanks a lot !