It’s pretty simple, though the author does leave out one crucial step:
You have to give Valve some money if you want anyone to see your stuff. If you haven’t already spent at least $5 with them in their store, you have to put at least $5.00 in your “wallet” (which you can do by clicking on your username in the corner of the Steam screen and picking “account details”).
If you do not do this, you will only be able to set your item/project’s visibility to “Friends-only” or “Hidden”. Once you add the money to your wallet or spend in their store, you can then set any number of your Workshop items to “Public” visibility.
I posted a couple of things on there just to test it out (it’s how I ended up on the hunt for information about changing content visibility), and I may list some more once NWN:EE is out of beta.