Enabled and configure Chatter Answer in Customer Portal.

Chatter Answer tab is viisble in Customer portal but while clicking chatter answer tab below error is coming.

"Insufficient Privileges

You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary. "

I have verified with user, user profile, Chatter answet setting but unable to find where is the permission setting is there.

Can any body help me on it.