2 How can I cite Octave?

Pointing to http://www.octave.org is good, because that gives people a direct way to find out more. If citation of a URL is not allowed by a publisher, or if you also want to point to a traditional reference, then you can cite the Octave manual:

       @BOOK{eaton:2002,
         author =     "John W. Eaton",
         title =      "GNU Octave Manual",
         publisher =  "Network Theory Limited",
         year =       "2002",
         isbn =       "0-9541617-2-6"
       }