For most Linux flavours and *NIX variants, the username you will connect with is root. However, FreeBSD AMIs are different. Ignore what the EC2 instance dashboard tells you when looking for instructions on connecting, which will look something like this:
ssh -i "your-aws-key.pem" firstname.lastname@example.org
Instead, you will want to change the username to ec2-user, as such:
ssh -i "your-aws-key.pem" email@example.com
SSH to the ec2-user and then you can
su to a passwordless root account.
Make sure you set a root password. It’s also a good idea to get rid of the ec2-user account, but create your own account and setup your SSH keys first.