Should  be a little hook like lever on one of the input pipes at...

Should  be a little hook like lever on one of the input pipes at the top of unit. Pull that lever up until you can hear the water pouring in, it should stay there. There should be an overflow pipe leaving the unit which probably goes outside somewhere?   leave the unit fill until there's water flowing from the overflow pipe, then move hook lever back to original position..water should stop filling. Hook type lever looks like image below.