There are many different ways to interact with Checkbox elements in browsers using HTML/Browser mode, using Accessibility Mode (or UIA mode if you are using 6.2 or later) and using Surface Automation.
For many of the interfaces you can send a click or press event, a global click event (make sure you activate the window first and have a tiny wait - as in the Surface Automation training on using global clicks or global send keys), you can use a write stage that sends ""True"" or ""False"", etc.
What options have you tried?