Itunes forbids the creation of folders or the drag and drop of a folder when transfering files.
My reccomendation would be to create the folder inside reactable mobile, then drag all your samples to the root folder in itunes.. Then from the Manage your sounds menu click edit (on the top-right corner in the tableview) and just select them all. You should see a checkmark on each sound file.
After this just click "Move" and select your newly created subfolder. All the files will be copied to that folder. It sounds like a long process but trust me, it's not more than a few seconds.
hope this helps.