Wiremold Ceiling Fan Box not compatible with Harbor Breeze mounting bracket. How to fix?
The Wiremold Solid Base Fan Box Fitting (part number V5783AF) is specifically designed for fans that have a fan bracket with holes on a 3 3/16"(81mm) spacing. If your fan bracket does not have this spacing then you will not be able to directly mount up the fan without doing some type of adaptation of modifications.  From looking at the Wiremold installation instructions it would appear that would not want to modify the Wiremold unit itself.





As you can see in the following diagram from the installation instructions that some fan mounting brackets have long slotted holes that accommodate a range of stud spacings.

  

This picture from a typical Harbor Breeze fan installation guide shows how the fan hanger bracket normally mounts to the electrical box using screws into the tabs in the electrical box. In the case with the Wiremold box the studs from the Wiremold back plate have to go through the slots in the fan bracket and then nuts are used to secure the fan bracket instead.



So if the slots in the Harbor Breeze fan bracket that you have do not have the 3 3/16" (81mm) spacing you will have to get another solution. Since it appears that you are committed to the Wiremold products for the electrical delivery to the fan site you will have to search for a fan that has a bracket that uses the correct spacing.

Note that it is not recommended to try to modify the parts shown here in an attempt to kludge together a solution. The safety of hanging the fan does require using the hardware as designed.