OK, let's see if I've got this. You've got a 0-10vdc signal on AI1 for a setpoint and your dancer signal also is 0-10vdc and is coming into AI2. You desire that the dancer operate in the middle of its range which is 5vdc on its feedback signal.
First, be sure you set the little yellow switches for voltage input rather than current input. These are located just above the control terminal strip and the factory default is voltage for AI1 and current for AI2. You need to change the second one to voltage. Also, you need to select PID Control macro in Parameter 99.02 and Vector-Speed in 99.04.
Before you attempt to set up the PID control, you need to run the drive and motor as a simple speed regulator and tune the speed loop in Group 23. Typical gains will be 30 in 23.01 and .5sec in 23.02. Usually 23.03 is left at 0.0.
Then you can switch to PID control and tune the outer loop in Group 40, Parameters 40.01, 40.02, and 40.03.
If you want the dancer to operate from the middle of its range then the setpoint input needs to be set at 5vdc. This sets up the neutral position for determining error.
Your mention of a dancer makes me think you might be doing a winder or unwinder with this drive and motor. If you do not have direct experience with tension control and winders, I strongly recommend that you hire someone to do this for you. They are one of the toughest applications to do correctly and your chances of getting it right are near zero. In fact, my chances are no better---I always defer winders to a system integrator that specializes in web process control.
Good luck.