Well to get an object out of catch 123d, you must remove the background by selecting it and deleting, a tedious prospect, then export it as an .obj. Removing the background is the hardest part to understand. Then I load that .obj into Netfab Studio (free) and re-orient it to top down if necessary, then export as STL. Don't know if an app is available for iphone like netfab. Also, you will need an app like netfab to cut off bottom if necessary, then repair to fill all holes, this closes the bottom so you have a closed mesh. After re-orienting, and repairing, then export as STL...
Also, your first catch in 123d will be in mobile format, good enough for tablet viewing, but you should consider re-processing it in 123d at Standard mesh, so it looks good on pc, and you will get a higher resolution mesh to print out.
The directions in Catch 123d are poor, almost like they don't want you to succeed....but i encourage you to try, eventually you will succeed! I encourage you to watch some videos on youtube re: Catch123d. Thanks for your interest.
