"Multipage" menus contain more than one menu "page" within a single psd (photoshop format) file. For a DVD they are converted to regular, single page menus, but can cause problems if you don't really understand what you are doing with them. You can have lots of menus - just use single page ones.
Every chapter already has a poster frame. Changing the poster frame does nothing for your link. The exception is a poster frame that is blank will not appear.