hello
With the keyword WIND GENERATION you choose the model to take into account the wind. The most simple might be to put a wind generation of 2 then the law is given page 23.
Then, if i understand well you want to specify a constant wind in the steering file. To do that you specify the keyword WIND VELOCITY ALONG X and WIND VELOCITY ALONG Y. that will be the velocity 'mesured' at the height specified by WIND MEASUREMENTS LEVEL.
Regards.
T.