Adding a Vacuum Tube library to OrCAD CAPTURE
Instead download a free version of circuitmaker 2000 here: http://getintopc.com/softwares/simulators/circuit-maker-2000-free-download/

Then get these tube spice models that have been written for audio enthusiasts here and install them into the program which is quite easy. 

http://www.diyaudio.com/forums/tubes-valves/84715-triode-spice-models-circuit-maker.html