Ls is right, based on the instructions:
Align bottle in brackets so that the valve outlet is pointing downward to the mounting surface (see illustration A) and tighten the bottle bracket bolts. NOTE: This is VERY IMPORTANT
so that the internal siphon tube will pick up liquid nitrous.
Also check out illustration A
http://www.nitrousexpress.com/Instructions/stage1.pdf
not sure of the purpose myself but in the pic it says for the bottle to be facing forward towards the front of the car.