Hi,
I thunk that Ubuntu decompresses these kinds of files in the GUI, just double-click on it.
If you really want/need to use the console, go to the directory where your .tar.bz2 is, then run tar -xvjf filename.tar.bz2 (tar -xvzf is for .tar.gz)
Code:
$cd containingdirectory
$tar -xvjf filename.tar.bz2
A new folder will be created in containingdirectory containing the decompressed content.
Serafean