sudo apt-get install unrar-free
if not, try unrar-nonfree by following the instruction next:
A,

1. Uninstall unrar-free.

   $ sudo apt-get remove unrar-free
   

2. Make sure you have a source repository by editing /etc/apt/sources.list.

   $ cat /etc/apt/sources.list
   # Default repository
   deb http://archive.raspbian.org/raspbian wheezy main contrib non-free rpi
   # Source repository to add
   deb-src http://archive.raspbian.org/raspbian wheezy main contrib non-free rpi
   

3. Sync the apt database.

   $ sudo apt-get update
   

4. Create a working directory and move into it. The unrar-nonfree command will be built in this directory.

   $ mkdir ~/unrar-nonfree && cd ~/unrar-nonfree
   

5. Install the dependencies required by unrar-nonfree.

   $ sudo apt-get build-dep unrar-nonfree
   

6. Download the unrar-nonfree sources and build the .deb package.

   $ sudo apt-get source -b unrar-nonfree
   

7. Install the generated .deb package. Its name varies depending on the version of unrar-nonfree.

   $ sudo dpkg -i unrar*.deb
   

8. You can remove the working directory you created in 4..

   $ cd && rm -r ~/unrar-nonfree
   

Once installed, you can use either unrar or unrar-nonfree (unrar is a simlink to unrar-nonfree).
Warning: unrar-nonfree and unrar-free options are different. For instance, to extract an archive:

$ # with unrar-free
$ unrar -x my_archive.part01.rar
$ # with unrar-nonfree
$ unrar e my_archive.part01.rar

Please man unrar-nonfree for more details.
B,