You'll want to make sure you're specifying the correct user and path to the key when you're logging into the Ubuntu instance via SSH.
The default user we create on our images is called: dhc-user
And the default path to the SSH private key is: ~/.ssh/id_rsa
If you're using the defaults, your SSH command should look like this:
ssh -i ~/.ssh/id_rsa email@example.com
(replacing 220.127.116.11 with your floating IP address)
If your key name is different, just replace ~/.ssh/id_rsa with the appropriate path to your key.
Hope that's helpful! Let us know if you're still having trouble.