November 2015
Trying to start SimCity 4 Delux from Steam on OS X 10.10.5.
After long timeout I get the following dialog box:
"This game requires Steam to run. Make sure the steam app is running, you are logged in its library, then run the game again."
I am logged in to my steam account and I could not try to start the application if i did not have it.
I do have the same problem with Sid Meier's Civilization V.
Rocksmith 2014 works fine.
To me the fault seems closely connected to Steam, but this is referenced to be the support page.
My steam ID is andreasmberger.
November 2015
To help you with this issue I would need to troubleshoot the Steam App, which sadly, I am unable to do. I would advise to contact Steam Support to see if they can be of further assistance.. sorry.
--Tom