Do you want to play as just a CEO, or as BOTH president and CEO?
If you want to play as a CEO and NOT a president: Goto Simcountry portal page, click on My Account (near top), click on "cancel a country", then select your country. (Warning: YOU'LL NO LONGER HAVE THE COUNTRY.) Then, same My Account menu, select "add an enterprise"... and go through the process of setting up your new enterprise on a particular world.
If you want to play BOTH a president AND a CEO: Goto Simcountry portal, My Account, "add an enterprise"...etc.