How to exercise with a torn meniscus?

No twist: Best to get it fixed. However, in the mean time, avoid twisting and pivoting activities as well as high impact. Stationary biking and elliptical would be best.

Listen to your body: You'll want to avoid motions that cause pain. Consider quadricep strengthening exercise but be careful about your range of motion. Avoid a >90 degree(right angle) bend in your knees, like if you are doing squats and/or lunges. Also avoid any heavy loaded squats or lunges. Be careful about rotational movements around your injured knee, like where you would plant and turn.