The error message you are receiving typically means that you have installed something that need legacy plugin enabled in your Joomla 1.5 environment.
Core CB and all CB Team released add-ons (for Advanced/Professinal members) are Joomla 1.5 native (and don't need Joomla legacy plugin enabled).
Something else you installed is causing this issue. You can try disabling everything else you installed one-by-one to find which one is responsible.