To perhaps fill in some missing pieces in the responses here: FreeNAS itself doesn't have the capability to mount Amazon Drive. In a jail, though, you can install just about any software that can be installed on any other FreeBSD system. So, you've found software that will apparently let you mount Amazon Drive on a Unix OS. If that can be done on FreeBSD (about which we have no idea, since none of us have done it), it can probably be done in a jail on FreeNAS. So, these are your steps:
- Create a jail (see @Sakuru's post for the link to the manual which discusses doing so)
- Find out how to install the software under FreeBSD
We aren't going to be able to help you with #2, as we don't know anything about the software. Hopefully the developer and/or other users of the software can help. Maybe there's even documentation (certainly looking for docs would be a good start).