Thanks
I now better understand what's happening.
The message:
"We are unable to retrieve your booking because this booking contains ticket(s) which is not issued by Singapore Airlines. Please check with the airline or travel agent you have made this booking with."
must mean that I am unable to Manage My Flight (at this stage Book the Cook).
I can check in through LH's site for the outbound journey (DUB-FRA-SIN) but guess that I will have to book the meals by phone.