Q&A problem

Hi support,
I am using Q&A Plugin for my multi-site. I want only registered user can post questions. How to set up that?
In the Question->setting
- Administrator
- Editor
- Author
- Contributor
- Subscribers
which one applied to my registered user? I allowed all type of the above users to View/ Ask/ Edit/Delete for question and answer. However, when my registered user login there is only one Q&A Search box.