Public Service Announcement
For some reason Atmel decided to hide the HID descriptor in the ASF code. If you create a new HID device, especially a generic one, it can be rather hard to find it. In fact, there are two places you need to edit to make changes to it. Naturally, to make the treasure hunt more exciting, they are buried 9 levels down the ASF directory tree.
src\ASF\common\services\usb\class\hid\device\generic\udi_hid_generic.c - contains 'udi_hid_generic_report_desc', the actual descriptor
src\ASF\common\services\usb\class\hid\device\generic\udi_hid_generic.h - contains 'udi_hid_generic_report_desc_t' which contains an array must be the same size as the descriptor