Hello?..................In the words of Pink Floyd "Is there anybody out there?"
Have I discovered an unsolvable problem here?
When sorted I will be happy to contribute real actual money to the authors of this otherwise excellent piece of software.
I also own and have paid for other similar mp3 tag editors in the past and this is definitely the best I have used to date (other than the problem I am trying to rectify).
Did you click on "Help" and read corresponding chapters?
When the OK button is not available, it means you have entered a pattern/string that does not match the filename.
Please look carefully that your pattern matches.
Simple example:
Filename: Pink Floyd.mp3
String: %artist% - %title%
-> It can't work because there is no " - " in the filename.