If you haven’t cut it yet, it doesn’t matter where you put the material you’re making the tag from. Those instructions were for the case where you cut the tag and then later wanted to engrave something on it, since alignment is best at the center of the bed.
When people refer to the camera, they are usually talking about the one that is on the underside of the Glowforge lid near the front. It’s the one that takes a picture of the bed and sends it to your computer.
If you want to add something, like name and address or whatever, you will need to create that in a separate program. The Glowforge does not have design capabilities, it’s more like a print driver. For bitmap content like photos and such, you would use a bitmap editor like Adobe Photoshop ($$$) or Gimp (free downloadable). For vector based graphics you would use something like Adobe Illustrator ($$$) or Inkscape (Free downloadable). You can find all kinds of useful information about these and other programs in the training matrix in the Glowforge Tips and Tricks section. The first posting has an index to all the training.