Select "Insert -> Check Constraint" from the main menu.
Right Click the Schema and select "New -> Check Constraint" then choose "with Selected Table" to auto-fill the values with the last selected Table or choose "without Selected Table" for a blank Check Constraint.
Edit the properties of the Table you want to constrain, select the 'Constraints' tab, select 'Check Constraints' and click the 'Add' button.
See Edit Table Properties Tutorial
If Editing an existing Check Constraint:
There are a number of methods to edit an existing Check Constraint in the Schema.
Choose the one you are most comfortable with: