Mac Mini :: New Folder Icon Grayed Out In Finder?

I can find the "new folder" icon, but it is grayed out in the finder window.  I am trying to add a new folder under the "administrator" log in on my Mac Mini and it won't let me and it won't let me under any user name.  I am trying to add a new folder under the "Public" folder. I have search high and low on the internet and have not been able to find an answer.  How do I turn on the ability to add a new folder? 

I have tried "read & write" under sharing and also under "get info", neither of these gave me the ability to add a new folder. 

I am sure it is a really simple answer as I can do it on my MacBook Pro, but can't on the Mac Mini.