In DKS most of the anti-dragon shields were removed, so the only generator that needs to be destroyed in Stone's fortress is the one by Xanlosch. If you are playing D2:ED / FoV, try leaving the fortress, maybe go to your battle tower or Broken Valley, return to the fjords, re-enter Stone's fortress and see if reloading that area will allow you to target the generator.
I assume you already tried standing nearby and using Whirlwind, Firewall or some other attack that doesn't need a target?