I just had the same problem. I decided to try something a bit different. It succeeded.
Step 1. Rename existing Sims 3 folder in My Documents\Electronic Arts folder
Step 2. Launch game thereby making a new sims 3 folder
Step 3. Install Greenhouse in new Sims 3 folder
Step 4. Move greenhouse file from library in new Sims 3 folder and paste it into Library folder in renamed Sims 3 folder (it will be the only file in the new Library folder)
Step 5. Delete new Sims 3 folder
Step 6. Rename old Sims 3 folder so its name is The Sims 3
Step 7. Launch game and place greenhouse from library into game.
It worked for me. Hopefully it'll work for others. Same procedure should work for any file missing from library. The trick is to identify which is the missing file and the only want I can think of doing that is to start with a fresh library. That means making a new sims 3 folder and installing just that one file in it.