Hi Bjorn,
Leave Management service was developed using Generic channel framework. So activating it under odata ICF node will not work.
To properly configure the Leave Management service please refer to http://help.sap.com/saphelp_gateway20sp06/helpdata/en/5f/3b1ed2f5dd4990ad722c5b8e5c9757/frameset.htm
Leave Management service connects to an HR system using Web Service. You need to configure Web Service as well.
By the way I hope you get the metadata when you execute the URL https://sapes1.sapdevcenter.com/sap/opu/sdata/IWCNT/LEAVEMANAGEMENT/$metadata
Cheers
Chandan