Madtoms are usually in deep cover. In streams they will be way down in the substrate. You can set a net downstream of you and shuffle your feet through the gravel or sand to stir them up. You really have to dig in- it's a good workout. In slackwater areas, you can net up weeds and leafpack and sift through them.
Unfortunately, madtom abundance tends to be real spotty. One site that looks good will yield nothing no matter how hard you try, while another will give you several madtoms in a single haul.
Madtoms are said to often hole up in discarded beverage cans. I've never found one this way, but it's worth a shot.