It's just not meant to be used that way, and despite the fact that there are usernames/accounts, the data itself is transferred in the open (i.e. unencrypted) and so is vulnerable to being intercepted. So I would highly recommend that you rethink your approach...

Personally I would look at secure FTP - either SFTP (FTP over SSH) or FTPS (FTP over SSL/TLS). There are many software solutions that can be used to connect using (secure) FTP (including an 'experimental' SFTP Windows Explorer plugin which I have tested briefly and found to be fine). There is also some software called SSHFS which has fairly functional Windows GUI port which I have been using for 6 months or so (and although there are some bugs, it's generally quite reliable).

The other (secure) option is to set up a VPN tunnel and then connect to SMB/CIFS/Samba through that. 

Having said all of that if you are hell-bent and/or don't care and/or it doesn't matter if the data is intercepted (and copied and/or modified) then AFAIK you can just open the relevant SMD/CIFS port(s) in the AWS security profile (AWS's firewall) then I would imagine that it should work...