If you have a mix of photos from different sources (e.g. with varying focal lengths) and not all photos were matched with a camera in the library, a grid is shown showing photo information. The cells in the Camera column each have a drop-down control allowing you to select the source of the Camera. This drop down becomes visible when you click on the Camera cell in the grid. Within this drop-down control, you have the option of 1) Browsing for a camera on disk to assign to the selected photo, 2) assigning a Default generic camera to the selected photo, or 3) selecting from a camera on in your camera library.
Do note that once your project has been set up, it is possible to import or create new cameras that you can assign to your photos. See Photo Properties for more information on camera assignment.