Once you have saved the file to your computer you need to import it into PT3 before you can use it. In the Configure Stats window go to the Statistics tab and click Import. Select the stat files that you saved. You should then see the stats and columns mentioned. Be sure to click Apply to save the new stats. However, if you can add the stats to the graph it sounds like you may have done that part, but then you should see the columns too...
From the stat description it looks as if that stat just uses a percentage of the rake, whereas you need one which uses MGR.
If you can now see the columns the easiest way to get what you want is probably to replace the rake part of the column expression with the MGR field, which is:
holdem_hand_summary.amt_mgr
Alternatively, duplicate the built-in MGR stat and change the new stat's Value Expression to "0.33 * amt_mgr".
If your rakeback is based purely on MGR then that should give you the right value.
Tutorial: Custom Reports and Statistics