I personally like to build using the behaviours rather than using the wizards as I get much more control over functionality and layout. This includes just creating my own forms, rather than getting any wizards to build them for me.
Then you can add the Server Side Validation for Unique Database Value. You then select the form field you want to validate (i.e the email address) and then you select your database connection, table and what field you want to check this against.
Then apply your error messages using the Show If behaviour that comes with Security Assist.
Does this help at all?