In order to use the DataAssist Delete Multiple Records server behavior you will need to make sure that you have the repeat selection properly applied to the form including the form element that has the id, this should be a checkbox.
Once the repeat selection has been applied adding the multiple delete should bring up the multiple delete server behavior interface. In here you just specify the table, the id column, and what element represents that column in your form. It should be a checkbox that has the value bound to the id column from your recordset. I'm including a screen shot of what the interface should look like for the multiple delete. Please post back with some more info about what you are experiencing and the result you are getting after the steps you are trying.