Say for example, you need to have to click an "I agree" box before being able to move to the the next page.  For this example, we will build a page that will link to another screen if the radio button is selected.  I'm assuming you already have the screen with a radio button already build, along with an additional page for which the user will move forward to after selecting the radio button.  

Once you have added a radio button to the page, configure to have a set name and a value.  

Next, after you have added your submit button, add a 'on click' action to it and 'Link to Screen', (or whichever applies to your project).  Once the action is added, you will need to configure the rules to use the 'Selected' value from the radio button. Lastly, make sure the 'Else' option is configured to 'Do Nothing'.

You can build on this further by configuring a hidden message, or image etc.  

If you have any questions, feel free to contact Support.