If you want to require customers to confirm their email address when they're signing up, all you have to do is include the following SmartTag in your checkout form:

[MM_Form_Field name='email-confirm']

In order for this to work correctly, make sure that you place the SmartTag between the [MM_Form type='checkout'] and [/MM_Form] SmartTags. After this SmartTag has been included in the checkout form, MemberMouse will require that this field be filled out and match what's in the email address field before allowing the form to be submitted.