When placing a connector you have the choice to place it on a face or place on a work plane in the options bar. if you select work plane it will bring up the work plane dialog box. chose pick plane and you will be able to highlight like when chosing to place on a face. this will put the center of connector at the bottom left corner of that face. you can then move it in the 2 parallel directions. What i mean by that is if you picked the top of a box to place the connector, you can only move in the X and Y not the Z.