To use flexible columns in the content editor, you need to add them to a page template first. Any modules you add to a flexible column in the design manager will appear in that column in the content editor.
Please note: modules added to flexible columns in the design manager will only appear on newly created pages. Modules can be added to existing pages from the content editor.
In your HubSpot account, navigate to Marketing > Files and Templates > Design Tools.
In the finder, click a template to open it in the layout editor.