I'm sure you know remmina is not included in RHEL 6. The only repository I found that provides remmina is "Russian Fedora Free repository". Of course it is not officially supported by Red Hat and pretty much "use it at your own risk" stuff. While the repository itself may not be malicious, the remmina packages I see in there are 2 years old.

Can you find an alternative that would do what you are trying to do with remmina? I understand it is a remote desktop client.