I can't find the predefined patterns that should exist (hope so :p)
directory /usr/share/inkscape/patterns is empty
Where can I find pattern files to install?

p.s.: I have installed Inkscape with synaptic in Ubuntu Feisty.

I don't believe Inkscape yet supports a pattern library. I think the folder exists purely because there is intention to create such a feature - but don't hold your breath. I believe this is also true for gradients.

yeah, the only thing we do 'stock' things for so far is markers, although the intent is definitely to do patterns and grads etc.
There are a bunch of patterns in OCAL that you can download tho (thats the open clipart library btw. http://www.openclipart.org/)