I am replacing an existing Moen tub filler, so I was extremely disappointed to not be able to do so with the parts included in the package.  As others have indicated, the "M-Pact Stem Extension Kit for Roman Tub" (Part #106378, SKU MOE2055) is required.  It costs less than $15, so I don't see why it can't be in the package or why Wayfair doesn't bring it up as a suggested accessory.