To extract RAR archives in Ubuntu, you need to install unrar.

On terminal run

sudo apt-get install unrar

Or you can install from Synaptic Package Manager

To extract a rar file, run terminal, go to the folder where the rar file is located.

Applications -> Accessories -> Terminal

Run

unrar x filename.rar

hoston@linux:~/my/share/software/FTP$ unrar x FlashFXP.rar

UNRAR 3.80 freeware Copyright (c) 1993-2008 Alexander Roshal

Extracting from FlashFXP.rar

Creating FlashFXP OK
Creating FlashFXP/FlashFXP OK
Extracting FlashFXP/FlashFXP/FlashFXP Website.url OK
Extracting FlashFXP/FlashFXP/flashfxp.chm OK
Extracting FlashFXP/FlashFXP/FlashFXP.exe OK
Extracting FlashFXP/FlashFXP/IEFlash.dll OK
Extracting FlashFXP/FlashFXP/install.sss OK
Creating FlashFXP/FlashFXP/Language OK
Extracting FlashFXP/FlashFXP/Language/Bulgarian.elg OK
Extracting FlashFXP/FlashFXP/Language/Catalan.elg OK
Extracting FlashFXP/FlashFXP/Language/Chinese Simplified.elg OK
Extracting FlashFXP/FlashFXP/Language/Chinese Traditional.elg OK
Extracting FlashFXP/FlashFXP/Language/Czech.elg OK
Extracting FlashFXP/FlashFXP/Language/Dutch.elg OK
Extracting FlashFXP/FlashFXP/Language/European Portuguese.elg OK
Extracting FlashFXP/FlashFXP/Language/French.elg OK
Extracting FlashFXP/FlashFXP/Language/German.elg OK
Extracting FlashFXP/FlashFXP/Language/Hungarian.elg OK
Extracting FlashFXP/FlashFXP/Language/Italian.elg OK
Extracting FlashFXP/FlashFXP/Language/Japanese.elg OK
Extracting FlashFXP/FlashFXP/Language/Korean.elg OK
Extracting FlashFXP/FlashFXP/Language/Polish.elg OK
Extracting FlashFXP/FlashFXP/Language/Russian.elg OK
Extracting FlashFXP/FlashFXP/Language/Spanish.elg OK
Extracting FlashFXP/FlashFXP/Language/Swedish.elg OK
Extracting FlashFXP/FlashFXP/Language/Turkish.elg OK
Extracting FlashFXP/FlashFXP/libeay32.dll OK
Extracting FlashFXP/FlashFXP/ssleay32.dll OK
Extracting FlashFXP/SN.txt OK
All OK
hoston@linux:~/my/share/software/FTP$