locking a specific setting in posts area

In the theme I am using, whenever I create a post, one of the options, way down the page, asks if want sidebar on the page. If I forget to check this then there’s no side navigation.

How do I hack this to be permanently on by default. I’ve tried hunting through the HTML and PHP and can’t find anything the gives me a clue. I’m not a programmer but have been aboe to change alot through trial and error.

Any idea where these value usualy reside?

Theme I am using is ‘Mission’