Meniscus tear in knee--is it painful?

Yes at times. Menicus tears can be painful but some can settle down and be tolerable for years if smaller. People complain of difficulty squatting, twisting, getting up from chairs or toilets, going up and especially down stairs, sometimes sleeping and turning in bed hurts or touching your knees together in bed hurts. Injections of cortisone can give temporary relief, arthroscopy and menisectomy is the fix.
Acutely, yes. Traumatic meniscus tears typically occur from a twisting injury to the knee and cause pain along the joint line. Mechanical symptoms such as catching, locking, popping may be present. A bucket-handle pattern tear can sometimes lock the knee. Degenerative tears often are not painful; MRI studies have shown they are common in asymptomatic people over 50. See a sports med specialist for your knee.