Several installs failed. As normal user than as admin, online & offline.
What do you mean by "installs failed" ? Have you got some errors messages during installation ? If yes, it's normal you don't see anything because, if the installer encounters issues, usually because of either your scenery.cfg or dll.xml files that were already corrupted before the installation, the installer will notify that, and will stop the installation, rolling back all files as if you haven't installed anything.
I downloaded ~300MB so i expect it to work.
Of course it work, otherwise you wouldn't much positive comments as we had until now (note that on support forums, you usually see only problem reports, but we are getting a lot of positive feedback instead)
No error messages, no messages at all...simply no buildings.
No messages at all, is not normal.
Assuming the installer completed successfully, when you first launch FSX, you SHOULD see TWO confirmation requests, one for the Addon Manager, and one for the Couatl engine. You have to reply Yes to both, this is also explained on the manual, with screenshots that will show you how the messages should look like.
If you haven't see these confirmation messages, maybe you encountered the dreaded Windows "lockdown policy" problem, that prevents any new executable to be trusted. There's a solution for this, please confirm this is the case.