Make a base for the arm by taking another piece of scrap wood that's slightly thicker than the thickness of the wheel and clamp the arm to the top of it. The bottom of the clamp should be under the lip of the table, holding the arm and base in position. You should now be able to slide a wheel underneath the arm with the axle hole facing up. Align the hole in the wheel with the hole in the arm and drop a dowel through the arm and into the wheel. You should now be able to spin the wheel in place without it moving around on the table.