It has nothing to do with not installing as administrator.
Open up the D:\Program files (X86)\FSX\PILOTS_Software\FS Global 2010FTX\settings.ini file and update the file path to terrain.cfg.
I noticed this line in the press release: "In the next few weeks they will also offer compatible installers for P3d and Steam's edition of FSX."
Does this still hold true for FSX:SE or are you comfortable that as long as we run FSX:SE only, there is no need for a Steam compatible installer?