You need a HPS magenetic ballast. You can not fire a CERAMIC metal halide bulb in a digital ballast (although I think there might be one if not more on the market now but I wouldn't risk it), nor in a metal halide ballast. Here's an article for you so you aren't getting it just from another newbie.
It's not about the fixture, it's about the ballast. All 150 watt CMH's that I'm aware of, are specified to use a "Probe start - Metal halide" ballast. Somewhere, on the package, or in the literature, listed under "Specifications", you will find an "ANSI Code" or an "ANSI Number"; that ANSI Code will determine exactely which type and size of ballast you should use for that particular bulb. This is the one I currantly have my eye on http://www.1000bulbs.com/product/4785/MHS-377200.html and it uses this ballast http://www.1000bulbs.com/product/7866/BS-EMZA00W150.html .