Copy Stand Alone World into Steam

Hi all.

I searched and could not find an answer to this, probably because I am weird. I own both the Steam and the Stand alone version of the game. I would like to copy my stand alone world into my Steam version, but when I do, the world works, but everything is gone. I checked the file and before I start the Steam game, all the files are there, but after I start and find myself in the wilderness, I check and most of the files have disappeared.

So, I am hoping someone knows how to copy an existing world from stand alone into Steam.

Thanks

Mike