the instruction leaflet talks about how to get to the screen to change wattage but never says hot to switch modes. that would definitely be a bummer if its a gimmick for the ill informed.
Hmm... Actually, while hard to read

... it looks like you can adjust by wattage as well. The "Switching the main menu" action requires you do press the main button 2 times within 1.5 seconds. Each time you do that will switch to another mode. The first mode which should show 'U' is voltage. And you can adjust up or down using the (+) button and the (-) button.
In the "U" mode, hit the main button twice within 1.5 seconds to get to the second mode "LCD". Here you can adjust the color of the LCD - pressing + and - will get you different colors. While in that mode - do the 'mode switching' deal by pressing the main button twice (within 1.5 sec) and you should get a "P" which is the power or wattage mode. Again + and - to adjust. The fourth mode is puffs.
The only thing is that it is unclear if doing the 'switching' mode while in another mode will take you to the next mode OR it may be that you have to press the main button 2 times and then another 2 times (to get to lcd) and another 2 times to get to "P" etc. OR if you hit the main button 2 times to get to U and three times to get to lcd, four times to get to U etc. I'm guessing it's the 2nd one - iow, pressing the main button two times (within 1.5 seconds) and then doing it again and again to get to P/Wattage.
It isn't absolutely clear but it appears that there is an actual variable wattage mode which is pretty nice...
Try it out and report back
eta:
OK... I missed your one post where you say it definitely isn't changing the wattage. If that's the case, I'd send it back or contact the vendor first and see what they say to do.