Post layout: Content width and sidebar

I feel like a total noob for not being able to figure this out, but I’m stuck.

Whenever I create a post (as opposed to a page) from scratch, the content fills only 3/4 of the width (see example link). I would like to insert a sidebar in the empty space to the right, but I can’t for the life of me figure out how.

I can only drag and drop a sidebar element to an existing row in the 3/4 content space. I don’t seem to be able to do anything with the empty 1/4 on the right-hand side.

I would like the posts to have a layout more or less like this: http://themes.kadencethemes.com/virtue-premium/image-slider-post/

Any suggestions?

Thanks!

The page I need help with: [log in to see the link]