The "Create a Case" option should be in the Actions menu only in your private customer support group. Go to the search box in the upper right, then search for your company name. Click on the group that comes up, and the Create a Case option should be there on the group Overview page. If you are not yet a member of your private support group, one of your colleagues can invite you. Cases vs Discussions in the Jive Community
Please let me know if you have any questions.