Your version of Photoshop Elements will not open D7100 images directly. Support wasn't added until Photoshop Elements 11 in Camera Raw 7.4.
Adobe stopped updating older versions to be able to read raw files from newer cameras whenever they released a new major version of Photoshop Elements. Thus, the version you have is no longer receiving raw file compatibility updates and will never directly be able to open your raw files.
You can check this yourself:
First you need to determine when Adobe released support for your new camera in Photoshop Elements. To do that, look at these two pages. You'll want to find out the earliest version of Camera Raw that can support your camera, then what version of Photoshop Elements can run that version of Camera Raw.
Since your version of Photoshop Elements cannot support your camera directly, you can download and install the latest version of the free Adobe DNG Converter, which can take your raw files as input and put out DNG format files, which your version of Photoshop can open.
The DNG converter DOES work, and you will be able to open the converted DNG files with your version of Photoshop Elements, but if you want maximal quality from your raw files (not to mention the convenience and ease of use of directly opening your raw files) you'll want the latest version of Photoshop Elements. Adobe has made substantial improvements in raw conversion quality in recent years.