First a few prerequisites need to be installed...
Commands
apt-get install make g++
Then download, extract the sources, compile and install it.
Commands
wget http://www.rarlab.com/rar/unrarsrc-5.4.5.tar.gz
tar -xvzf unrarsrc-5.4.5.tar.gz
cd unrar
make
install -v -m755 unrar /usr/bin
-i