In Chapter 8, "Arena", defeat the Steracosaur Boss. After doing this, the force field around the middle pillar will drop. Climb to the top of the pillar to get both the Nuke Attachment (for the Rocket Launcher) and a Nuke. You will now be able to fire the Nuke only twice (the Nuke Attachment has a Nuke already in it). There are no more Nukes past Chapter 8. Also , the "All weapons" and "Unlimited ammunition" cheats do not work for the Nuke.