Repeater is an extension for sending (repeating) PDUs. Each PDU has its own tab that tabs consist of two sections - PDU editor/viewer and History view.
You can easily send any PDU from History to Repeater or create your own PDU using the Create button in the top right corner of Repeater tab.
You have start PETEP (Settings tab) to see the PDU editor, otherwise you will only see PDU viewer. If you only see PDU viewer after PETEP starts, you have to use the Fix PDU button to choose proxy for the PDU. (This might happen when you change proxy code or remove proxy from your project.)