You will need to install that database option from the installation CD or whatever.
Run the installer and find the parallel query option and install that.

Once you have that setup, you need modify the init.ora file to include sufficient parallel query servers so you can use all 6 CPUs. Have a read of the doco on parallel query.
Bounce the database to get those changes and run a test to see whether you can use parallel query (using hint below). You should see processes (on Unix) called ora_pxxx or something similar which will indicate that parallel query processes are available and possibly in use.

Unless you make you tables /partitions parallel at creation you will need to hint to use parallel queries.
/*+ PARALLEL(degree n) */
again the doco is pretty good in this area. Look under the chapter on TUNING.