When you have registered inside MACE, you can add new pages by using the Bookmarklet. Drag it to your browser shortcuts (bookmarks) and then visit any site you want to add. By clicking on the bookmark, a small script will give you an upload form, where you can add page title, url, short textual description and the main language.
After that, the page is fully entered into the MACE system. You can tag and enrich it as any other item.
If you try to bookmark the same page again, it will just forward you to the existing instance.