PX4D mid range adjustment?

[Gtr76]

hi mates!
new to the forum!
well last week i bought the PX4D and i love it so far...
but i can't arrange the mid range on the eq.
there is no button and in the manual write (p.25) that can be adjusted with the value dial?
i can't get it....
in the button which is up and far left writes gain/mid
how can activate it?
thanks and sorry if it's covered in another topic!