The wattmeter uses DC; what you read is probably the rating for an AC 'wall wart' power supply. If you are running a station along with the meter, then I would just run a power lead from your DC power supply to the meter. Just be sure of the polarity of the plug on the meter's end. For some reason, some of the meters use a coaxial type power plug with the center positive, (considered the 'normal' configuration) BUT some units had the center negative, and the shell positive, as K0RS mentioned. There was a sticker on the back of the meter warning if the center was negative, but it may not still be there. To further confuse things, some of the meters used a mini phone plug, with the tip positive. Be sure of which yours has!
Once you get it in line, you will find that the meter works very well, and having immediate power and SWR readings on the two meters is a fine feature. I find it easier to read the SWR that way than with cross-needle type meters. (One of the ones I had used the center positive, and the other had the phone plug!)