Sometimes ,I download buildings from simtropolis ,they would show up as a huge gift box on the game.Why?

Make sure you have the model file and desc file or sometimes the dat file in the plugins folder. The model and desc file are needed for the building to show up. Of course there some buildings without the model and desc files just the lot because some of them forgot to upload them, like me for my first building uploaded. :D

Did you download the Montreal Stadium or the Polamile Centre?Where can I get their missing files?

Did you download the Montreal Stadium or the Polamile Centre?Where can I get their missing files?

Nope I didn't. What files did they have? It's either a DAT file or 3 or more files inside. Read the comments, did they mention anything about missing files or did the author himself say anything about forgetting to put files?

No.

Basically wne you unzip a file you need to insert every file that is not a photo file, document, or another zip file. i think that covers all the things you don't need.

Yes make sure you unzip every single file included including the ones from those inner zips. Some people place zips in zip files.