How to control the voltage coming off an AC source?
The proper way to control the output voltage of the linked LCC25 driver module is to connect it to your Arduino with an RS232 cable and give it commands as described in the manual of the LCC25 liquid crystal controller.

Your proposed "solution" can change voltage, but only for "loads" whose resistance is many times that of your voltage divider.  You could probably measure the voltage properly with a good multimeter, but you couldn't draw enough current to actually do anything useful with it.

You also have the problem that digital potentiometers can only handle input voltages within their own supply voltage.  So, not going to work if you have +-25V and the potentiometer is running on 5V.

Just use that supply as it is designed.  Talk to it over RS232, and tell it what you want.



I see in the comments to a deleted answer that you need to be able to control it faster, that the USB commands have too much latency.

Controlling it through an Arduino and a digital potentiometer wouldn't be much (if any) faster.  

Controlling it through RS232 won't be any faster.

The controller can be programmed to modulate its output.  If you know how it needs to be modulated, then you could set it up beforehand.

If you don't know how you need to modulate it beforehand, then you'll have go another route.

But, there's not enough information in your question or the comments to really explain what needs to be done.

You would need to explain what tells you when you need to change the modulation, and how.