Sceneskies Hdri Pack
How do I use these images to light my 3d scenes?
The most straightforward way is to use them via their accompanying addon, get it from its website, it's free.
Read the documentation to know how to link the HDRIs you bought with the free addon.
Is this an addon?
Short answer: no, it's a collection of images for lighting your 3d scenes. (HDRIs)
So you DO NOT install the zip files you get as if they were addons, because they're not.
Long answer: there is a free addon made specifically to easily use this pack, download it from its website.
The addon makes your life easier, and is the only practical way to use the color-tweaking feature of this pack (see this product's "About" page to know what that feature is).