I can get it to work with a dropdown list selection, but not with a selection from a checkbox list.
I'm a developer with Cognito Forms.
You would have to set up a Show this Field 'When' condition. You would need to target the Choice field on your form, in the first drop down list. Then select 'Is' from the second drop down list. You would then need to enter in the value that your looking for exactly as the value in the Choice field. As an example if you are looking for the option 'Second Choice' you will need to type the option exactly like that, same capitalization as the Choice field option.
Here is a screenshot of what this will look like.