This issue may be caused because of the version of Unreal you are running. We try to keep our packs up to date with what is currently out from Epic but sometimes people will have started a project further back than what our files are made on. Currently our packs are version 4.10 and forward. If you have an older version you can open/create a scene in 4.9 that has the SK_Mannequin in it, then select and import all the FBX files included in the "Source" folder (No mesh, choose the SK_Mannequin skeleton and animation only). You can select and import them all at once.
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