valliant turbomax plus 824e/low preasure?

how do i fill up the system so the preasure goes up to normal cos i cant find the valve to do so, i found one water inlett valve but it only turned the water inlett for the hot water outlett on and off, HELP!!!!!!!!!!!!!! preasure at 0.2 nearly 0 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!