It's AC, but with some devices optionally earthing one terminal to the frame (a hang-over from the days when wire was expensive enough to make it worth only running a single core cable).
A potential gotcha is when the dynamo connects one terminal to the frame (as most Shimano ones do), and a light connects the other, so current flows through that path and shorts out the dynamo (which won't cause damage, but will prevent the system from working). Convention is that anything with an electrical earth symbol, and (on B&M lights) minus symbols and black wires with white stripes should be connected together.
But I don't think that's your problem, since there's no rear light - I'd expect the Luxos's mounting bracket to be electrically isolated, so the polarity wouldn't matter.
Have you actually ridden the bike, or just spun the wheel? The light may need to charge its battery/supercapacitor (not clear if this is a Luxos U or B) for some tens of seconds before it starts to work. Spinning the wheel by hand is surprisingly feeble in comparison to riding at even modest speed.
FWIW, the Luxos manual is here:
http://en.bumm.de/fileadmin/user_upload/179/179_LUXOS_B_U_Anleitung.pdfAnd in case you've cocked it up, wiring a Shimano lego brick connector works like this:
Separate the two parts of the connector, insert the stripped and twisted wires into the grey part, and fold back into the groove, then click the grey part into the black part to secure the wires. It then push fits onto the hub terminals.
If you have a multi-meter, the dynamo should measure a couple of ohms on the resistance range when not turning, and you should be able to measure some tens of volts on the AC voltage range when you spin it.