Enabling the Theo Price on your DOM is the theoretical average on the current trade until you flatten the trade.

For example,  if you enter at a price of 100.00 with 2 lots and you scale at 100.10. At that point the theoretical average should be 99.90 (0.10 away from entry point). If you add 1 again at 100.05 and scale it again at 100.10, then theo should update to 99.85.

The way to enable this on your charts is by going to the Cog wheel in the top right corner of the DOM. See below for clarity on this: 

      

After this you will want to locate the, "Price Column" tab and click on it. See the image to better understand this: 

      

The last step is to check the box that is next to the, "Theo Avg." Look at the image below to better locate the box that needs to be checked. 

       

Be sure to click, "Apply" then "OK" after checking the box to officially add this to your DOM. 

      

You should now be able to see the Theoretical Average when you enter and exit trades on your DOM.