Hello Kevin,
I solved it by using LOIWCS02 IDoc type in the partner profile. It seems to me that in ME 15 two possibilities are covered:
If you want to work with normal resource / work center concept in ME (WC category 9100 or 9000 in ECC) then you need to use LOIWCS02. The workflow for resource and WC is maintained for LOIWCS02 IDoc type only
If you intend to work with individual capacity assignments (split operation) then you need to work with LOIWCS03.
Depending on your business case, you may decide to use on of them.
Thanks
Srinivas