Honestly, I think Metrolinx is right regarding the UPX. The lack of visibility/usage is more of a function of engrained travel habits than anything. When visiting London, for example, people are already aware of the Heathrow train and will seek it out. Toronto on the other hand, has been well served by airport limos, and that's what people are going to continue seeking.
The UPX should have been free for a month or two in order to get people onto it, since the best marketing they've got is the train itself. Simply having signs saying a train exists won't help change people's travel habits. They need to experience the quick, smooth ride in order to understand it. I guess you can have reps at the terminal actively trying to sell the train, but who really wants to stop and listen when you've already decided that you are looking for a cab?
The only way they are going to increase ridership is through word of mouth. Once enough people have used it and are talking about it, word will spread that the service exists and how efficient it is. That will drive people to seek it out, at which point it will be easier to sell.