Adding templates to mail software in OSX Lion
These stationery files must follow strict Apple rules, meaning they have to be created in certain ways. If you bought them from Apple they will be fine, however, they should have told you where to place those files. 

Having tried unsuccessfully to create my own, I at least learned where to  keep them. Here is the path where the files should be placed:

(your) Hard Drive:Library:Application Support:Apple:Mail:Stationery:Apple:Contents:Resources:Photos:Contents:Resources:Air Mail.mailstationery

Air Mail is just one of the Apple stationery packages. Yours might be called Thai Forest.mailstationery.

Hope this helps.