Could not open *.rar file in Ubuntu
 

The default archive manager in Ubuntu could not unzip rar file,
how to enable it?

Try RAR isn't a free-libre format, so you need to install it manually :)

RAR is available too : sudo apt-get install rar

But : "This program is shareware and you must register it after 40 days of use."
http://packages.ubuntu.com/maverick/rar

Anyway, installing unrar, as said by @ Snark1994, will do for your requirement.

Can I install it by Synaptic Package Manager?

Yes, it is in the repos. I just checked.

Here's the developer site:

http://www.rarlabs.com/download.htm