I get a message saying that it is the wrong type of file. Any bit of advice?
The Nikon D5100 did not exist when Elements 5 was released in 2006. Your D5100 was introduced five years later in 2011.
To open RAW files from your D5100 you will have to update to a more current version of Photoshop Elements. The earliest version of Adobe camera RAW (what PSE uses to process RAW files) that supports the D5100 is ACR ver. 6.4.1. The first version of Adobe Photoshop Elements that supports ACR 6.4.1 is Elements 9.
You could also open the RAW .nef files from your Nikon D5100 in the software that came with your camera, ViewNX 2, make basic adjustments such as exposure and white balance, and then export them in a format such as JPEG that PS Elements 5 can open.