The only photo of it that I could find was not really detailed. From the photo, it looks as though the pressure gauge is horizontal on top of the moisture regulator which is where one would normally expect to find the regulator knob, clockwise for more pressure, anti-clockwise for less.
Their website did list some contact e-mail addresses. Perhaps they could help, send you a PDF or something.
http://www.promotion.com.tw/t-39_noise-free_mini_air_compr.html