I can't add any pagelets to a page

If a page is set to have the Sidebar hidden then any pagelets you try to add to it will not appear. This is because the pagelets are being added to the page, but into the invisible Sidebar!

So, to add a pagelet to such a page, you need to make the Sidebar visible again (using the Page Inspector). You can then either keep the Sidebar visible, or set some pagelets as callouts.