Thank you Ray, now It works!
Now the plugins work. So when the user presses respective button - the content from the plugin is displayed.
I have two questions:
1. No content from the plugins will now appear until the user presses any of the associated buttons. How do I do if I want the content from one of the plugins to be displayed by default - when the page loads? It should still work as before, the content should change depending on which button is pressed.
2. Both plugins contain links to another page. When this (linked) page is displayed, an error message appears when you press the back button in the browser. It says:
Confirm Return Forms
This web page requires information that you have entered previously in order to be displayed correctly.
You can resubmit the information, but if you do, the actions performed by the page are repeated.
To resend the data needed to load the page, press the Reload button.
How to solve this?