How do I install new themes for keynote purchased throught the app store?

I have downloaded some Keynote themes fron a site but I have no way (that I can see) to install them. I've done extensive googling and I always find instructions that say that I have to add the downloaded themes (.kth files) into the ~/Library/Application Support/Keynote/Themes folder. However, I have no such directory.

I purchased Keynote via the App Store. Could this be the reason why I don't have this directory on my hard disk? How does one install new themes then?

Thank you.

While I am at it and since there must be a lot of Keynote experts around, I'll ask another question about a problem that is driving me crazy. I'm one of those thousands of new converts and I have a lot of old PowerPoint files that took me a lot of time to create. I'm trying to import them into Keynote but some of them are giving me trouble. The problem comes when I try to change the theme (I just want to change the background). When I have complex slides involving many objects, if I change the theme all these objects abandon their original positions and everything becomes garbled. I think this has to do with the aspect ratio of the slides but I haven't found any way of fixing it. No matter what size I choose in the Theme Chooser, the result is the same.

I would really appreciate any help you people can offer with any of these problems. Thanks in advance.

JM