To add a federal extension request using IRS Form 4868, open an existing 1040 return;

or create a new 1040 return for the taxpayer(s) you wish to file an extension for.

Once the return is open, find the State/Extension drop-down box in the navigation pane and expand the drop-down box.  From there, find Federal Extension and left-click on it.

This will open the extension similar to how a state would be added.