Don Johnston Tools/Extensions interact with the webpage/tab you are using them on. Without having access the Extension/Don Johnston tool will not appear on the webpage/tab.
Examples of the "This can read and change the site data settings":
When you click on the extension:
You will need to click on the extenion icon. Chrome will then ask to reload the webpage/tab so the extension can see the webpage/tab.
(If you have this setting set for multiple extensions, Chrome will have to reload the page for each extension. )
On Current website:
You will have to allow each extension access to each individual website/page you visit for the first time with the extension.
(Chrome will also ask to reload the webpage/ tab at this time to give the extension access to it.)
On all sites:
The extension/tool will work like it normaly does.
*Recomended setting for Don Johnston tools is having '"This can read and change the site data" set to: On all sites
Steps on how to set this setting can be found at: