Hi, you don't need to do anything to decrypt the files; if it's encrypted SSave will already know--it is handled automatically (using Sphinx internally).
As of V1.4.0 you should be storing your inventory data as either a struct or an array. But, I will write down a note to add support for more data structures as it would definitely be useful.
Are you sure you're adding the files to the import list when opening the package? GameMaker doesn't select anything by default.