For booking retiring room you have to go the retiring room site of irctc tourism.
Go to this url: http://www.rr.irctctourism.com/cgi-bin/rr.dll/irctc/services/rrhome.do
Then login from your IRCTC username and password. If in case you are unable to login then try login through guest panel. Make sure you enter the same email id and number as you have used in your online irctc ticket. See image below:

Hope this helps you book your retiring room.