Hiya,
If you hover over that checkbox you'll see the message "Cannot show these buttons when break buttons are only shown in the main toolbar".
So what you need to do is:
- Select the "Display" options
- Change "Show break buttons" to something other than "Only in the toolbar"
- "OK" the options
- Open the Options again and the checkbox will be enabled.
I have to admit that isnt very intuative ;)
Oh, and ZAP has not been an OWASP project for nearly 2 years now :)
Cheers,
Simon