Determine the version of the .skel file then run Spine that is that version and import the .skel file into a project.
http://esotericsoftware.com/spine-import
You can look at the .skel file in a hex or text editor. The version is near the beginning. If it's not, the version may be very old.
Next use Spine's texture unpacker to unpack the atlas into individual images. Change the images path of the skeleton in your project to the folder containing those images.
