If your vehicle isn't on GM's current list, don't bother with this app.
GM has absolutely no interest in catering to customers who bought one of their vehicles previous to 2011. There are only a handful of 2010 vehicles on the list, mostly the more expensive SUVs, which tells me there isn't a technical issue, just no financial driver to promote the app to buyers of their most popular vehicles (their trucks).
Don't let some GM PR spin doctor try and tell you any different.
Here is the statement on the current FAQ for the app:
When will the OnStar MyLink mobile app be available for previous model year vehicles?
The MyLink mobile app is now available on select 2010 model year vehicles as well. At this time, there are no plans to extend the app to additional 2010 or earlier model year vehicles
Hey, it's GM. What did you really expect?