To Add a Spouse During Profile Creation:
Click "PEOPLE" on the left sidebar.
Click "ADD PERSON".
After filling out the profile information, click "ADD SPOUSE/PARTNER".
To Add a Spouse to Existing Profile:
Click "PEOPLE" on left sidebar.
Select the profile you wish to add a Spouse/Partner for.
Click "ADD SPOUSE/PARTNER".
After filling out the updated profile information, click "SAVE".