Look for the folder /Portals/_default/Skins/DNN-Blue
and folder /Portals/_default/Containers/DNN-Blue
That's the default DNN skin with matching containers.
Now log in as Host and go to the Host, Skins menu and look for DNN-Blue on the list of skins. That's the skin from the folders above. Any skins you install should follow the same structure.
Ideally your skin will be a (skinname).zip containing two more zips, skins.zip and containers.zip.
If it does then log in as Host, go to Host, Skins menu, hover your mouse on the little downwards pointing triangle and you will see "Upload Skin" (bad workflow logic, I know). Use that to upload your (skinname).zip. Watch it upload and watch DNN 'parse' the skin.... that means it usually takes the html files in the zips and coverts them into the ascx files that are the actual skin. It also puts them in their proper folders. When this is done, you should see the new skin on the Skins drop down menu where you previously found DNN-Blue. You should always use the Preview rather than the Apply option when checking a new skin, because a bad skin might cause your site to be inaccessible.
You do not need to use the "Upload Container" menu option unless you have a zip that consists only of containers.
When using the Host, Skins page, your skins will upload to /Portals/_default/.
When using the Admin, Skins page, your skins will upload to /Portals/0/ instead.
The skins in /_default/ can be provided to all sub-portals of the DNN site, whereas the skins installed to /0/ are only available to portal "0". Portal zero is the default sub-portal and every time you add another sub-portal, a new folder is added with a higher number /1/, /2/ etc. A sub-portal is what DNN calls parent and child portals. Just think of them as extra sites running off the same backend.
If you have uploaded the skins by ftp, and they were html-based skins, then they will need to be manually 'parsed' to convert them to ascx files. When you have selected the skin in the Skins page, click 'Parse Skin Package' at the bottom of the page and watch it do its thing. Then you can Preview and Apply the skin. You must parse every time you make a modification to the html skin files directly on the server.
And don't worry if it all seems ridiculously complex and unintuitive. That's because from this side of the application, it is.
Good luck,
Rob