Overlays are not actually installed in Photoshop Elements. They are actually imported as an image and used along with the image you are editing.
Download your Overlay file and ensure it is unzipped and ready to use.
On Windows double click into the folder, select all, copy and paste into a new folder outside the zipped folder.
On Mac simply double click into the zipped folder and Mac should extract the files into a new folder for you. If you downloaded via Safari the zipped file may be unzipped already.
For organizational purchases it’s best if you can save these unzipped files in a folder together named by collection. For instance, I would title my folder “LLP-HappyHour” and place all of my Happy Hour Collection presets in that