Amazon ec2 and cyberduck pem file

Amazon ec2 and cyberduck pem file

I am so close to figuring this out.
I've set up an ec2 instance and can get in via ssh through terminal (Im on
a mac).
I want to be able to get in via cyberduck and sftp, and can with the
ec2-user@mydomain.com login but then I don't have sufficient privileges to
actually do anything.
When I try to login via cyberduck with root@mydomain.com it won't let me,
says
Illegal sftp packet len: 1349281121.
I created a new user but don't know how to get a .pem file uploaded for them.
Thanks for helping a first time linux user.