cannot add categories (or Amazon posts)

when i try to add a category using the Posts -> Add New screen, it doesn’t do anything. clicking on Posts -> Categories then typing in a category name and pressing Enter returns a page that says Oops! This link appears to be broken at the address ______wp-admin/categories.php using IE. (it showed up as an Error 404 on my laptop using the newest Firefox).

also, when i try to use Amazon Autoposter to add a post, it says “No result found. Skipped category link”.

any help would be greatly appreciated! this is my first blog and i’m kind of lost.