Sorry Ken and all.
A DER is a Designated Engineering Representative, an individual designated by the FAA, and given the authority to approve data for repairs or alterations to an aircraft or engine. The FAA has their own engineers at the regional level who would/could approve data submitted by individuals or manufacturers for STC's (Supplemental Type Certificates) which are changes to an aircraft or engine TC (Type Certificate) or Field Approvals on a FAA 337 on a one time basis. Often times it takes a while for the FAA to get to your request, to process it, request changes, wait for changes to be submitted, approve or disapprove, anyway you get the picture. This is where DER comes in, he/she may approve the data, or generate and approve his/her own data which, which then meets the requirements for engineering review by the FAA and approval is sped up. Of course you have to pay for this, where as it is free from the FAA.
Aeroski has been waiting a couple of years to get approval, it may be that the FAA just doesn't have anyone with expertise with ski installations. So for them the trade off is, will there be enough interest to cover the cost of hiring a DER to get approval.
Sorry for the long post, Tim