The Content Box is something you have to first add via the Store. Go to Dashboard > PageLines > Store > Sections > Get New Sections > Top Free and click Install under Content Box. Once installed, you may then add it to the sidebars in Drag & Drop.
However, you may also apply a text widget (as many times as you wish) to any sidebar via Dashboard > Appearance > Widgets. Anything can go into text widgets and using Widget Logic (a free plugin found at WordPress.org), you can set where these widgets (or any others) will appear on your site.
Personally, I don't recommend using the Content Box on a global basis as it has the ability to be set in individual pages using the PageLines Meta Settings. Using it globally would prevent you, potentially, from using it elsewhere if you want to place it on other pages or posts.