Install or Save a Material Library Fails: Permissions
This is likely a problem with permissions - Kerkythea, when running under your user account, does not have permission to change anything in the Kerkythea program folder... such as creating a material library.
It is not too difficult to give the permissions - you don't give permission to Kerkythea, but you give permission to the User Account on the machine.
Right-click the Kerkythea Program folder (probably found here: C:\Program Files (x86)\Kerkythea Rendering System) and choose "Properties" then follow the steps listed here but you probably need to do this for your User Account, not for the Administrator account:
http://www.wikihow.com/Change-File-Perm ... -Windows-7
Use google to search for your computer system if you are using Vista or Windows 8 or 10 or other.
I'm attaching a pdf print here for posterity.