I think the resistor is only to adjust the highest possible speed of the spindle.

I would try to connect a rheostat (or potentiometer, if thats a common word in english)
to connect between the 10V and 0-10V clamps of the iso-card. Then the spindle speed must
change with the position of the slider. If this works, then you canconnect the 0-10V output from the controller board to the Iso-card. After that you must connect the Rel2 to the hot-wire of the lenze drive and connect Rel3 as a commutator between drive and motor.
Attention, you must wait until the motor standstill and half a second or so, before you can switch the direction.

In my machine I don't have any home or end switches, but I need to assamble some, it where better I can add some linear encoders, but that will last some time.