Normally +r means that the channel is registered, but +R means that you have to be using (and identified for) a registered nick before you can join the channel.
There are some networks that just use +r rather than +R, but it does the same thing.
1) Make sure that the nick you are using is a registered nick.
2) Make sure that you have identified to the nick registration service (normally nickserv) as the owner of the nick using the appropriate password.
3) Attempt to join the channel in question again.
If this does not work, please provide the network information required to connect to the network so that we have the ability to check the specifics of that network.
