Normally, the base of the arm platform has about a 1" diameter brass tube that is threaded, and goes through the plinth. There is a large nut that screws onto it, and secures it to the table. The cable exists through the bottom. If there are not any threads on this, then there is some other kind of modification done to this arm. I have no idea what to suggest in this case. Perhaps you need to ask the guy you got the arm from how he secured it to his table.