Help! Books only appear in My Folder

I used Calibre to transfer all the ebooks to my iriver. However, when I open the device, none of the files are listed under 'books' at all.

I found all the files I transferred under 'My Folder' instead.

The problem with this is the files in My Folder can't be detected via search function and I can't even bookmark the files there during reading.

How to make the books appear in the general 'books' category?

Thanks.