I think the Construction Helpers only work in the Constructor of a class.
It is bad practice to hardcode references to assets. In case you want to do it anyways write this in the constructor:
static ConstructorHelpers::FObjectFinder<USkeletalMesh> Mesh_Male_FeetObj(TEXT("SkeletalMesh'/Game/Characters/BodyParts/Male_Feet_LOD_0.Male_Feet_LOD_0'"));
if (Mesh_Male_FeetObj.Object != NULL)
Mesh_Male_Feet = Mesh_Male_FeetObj.Object;
“Mesh_Male_Feet” is a USkeletalMesh-pointer (declared in the header).
You could also just make the USkeletalMesh-pointer and specify the asset in the blueprint… I guess this would be the “right” way to do.