I used this brand on my D3100 and now I have it on my D7000. I'm very happy with it. For the D5100 there may be an issue in closing the viewer towards the body. Read the reviews but the product itself is great. Amazon.com: GGS Optical Glass DSLR LCD Screen Protector for Nikon D5100: Electronics
I haven't had that problem. Make sure the screen is mounted with the wider border on the bottom and turn on the LCD light while applying it. There should be no overlap of the border on to the screen information.
Let us know how it goes on your 5100 thanks. Regards Phil