Ubuntu Archive Manager can not open rar file by default in your Ubuntu. Because of the proprietary issue rar support package not included in Ubuntu distribution. However rar support installtion in very easy with apt-get package mamager.
Follow these steps to install Rar Support in your Ubuntu Linux Distribution.
Open Terminal Window and Run the below command. You may ask to enter admin password if you are not login as root.
sudo apt-get install rar unrar
The above command will install rar/unrar capability in your ubuntu linux box. You should now able to UnRar archives using Archive Manager.