Problems with HTTP_REFERER.

Hi
When i want to delete or add a categories,this message appear:
Bad HTTP_REFERER. If you have moved these forums from one location to another or switched domains, you need to update the Base URL manually in the database (look for o_base_url in the config table) and then clear the cache by deleting all .php files in the /cache directory.'What can i do?Please help me,i am speak a little english that why i want u show me where i can change the code.Thanks