1.) Basically add the three folders (OTHH, CITY, TERRAIN) manually to your scenery library within p3dv5.1.
2.) Download the patch file posted in the inibuild forum: https://forum.inibuilds.com/topic/18-prepar3d-v5-scenery-compatibility-files-othh-error/page/2/
copy the patch file to your OTHH scenery folder (if I Remember correctly).
Optional: You can also download the GSX Profile and dynamic lighting from inibuilds.