This is a known issue with CB Content and we've fixed it for next release. You can review the bug ticket at the following URL.
forge.joomlapolis.com/issues/2161
Currently there are many known bugs with CB on Joomla 1.6.x. I do NOT recommend using Joomla 1.6 until we've released the next version of CB which has such issues addressed.
Backend menu display is due to a bug within Joomla 1.6.x that was never resolved (despite our attempts at communicating with Joomla team and bug reports). Many extensions feature the same issue.
Folks Joomla 1.6.x JUST released. Please do not expect CB or any other extension to work on it. Developers have not been given ample time to make adjustments as necessary; some extensions require complete re-writes. While we're working as quickly as possible I do not recommend using Joomla 1.6.x for another 1-2 months.