When that happens, it means the graphic you have selected is already considered a teleport pad. If you copied an existing configured teleport pad, it seems to get confused, so drag in something to use for a pad (a Prototype Image), and make copies of that unconfigured prototype for each additional pad, then select the copy and click the "Create Teleport Pad" button. When linking, make sure you are selecting the destination pad, then clicking the chain link icon for the source pad in the menu. For a bidirectional system, you'll need to set up the relationship both ways.