In order to add a new spouse use the EDIT button on the genealogy card of the person to whom you are adding the spouse.
Go to Spouses -> Add Spouse. A window will appear where you can add the spouse's name and details.
When finished, click ok. Go back to the genealogy card of the person you added the spouse to. You should now see two spouses listed. You can click on the spouse name in order to switch between them.
**Try using the Manage Spouses screen in order to arrange marriage priority, add additional information, etc.