Leonidas-the nixie tube needs at least 150 volts, 120 volts will not work. It is best to use around 170-200 volts for reliability. In order to get this voltage, you must construct a boost circuit. This is from my site documenting the nixie clock I am working on, http://sites.google.com/site/willyager/nixie-clock.
I believe an older (but working) version of the code is there, and so is a picture of the actual circuit. I have run it from both 9 and 12 volts with no problem (I don't think 5 volts will work) and it is stable with 1 or more mA of current draw. It is really just a standard boost circuit with some custom code to regulate the voltage to what I want (right now it's at 166 volts, but higher is probably better).