Installing New Garbage Disposal - leak at P-Trap?

Installing a new InSinkerator. Have everything hooked up properly (to the best of my know how anyway). Issue, the discharge tube. Where it is supposed to hook into the P-Trap. When I run water into the drain, there is a leak where the discharge tube and the existing plumbing meet. Questions: What can I do to stop this leak? If I try to tighten the connection, the discharge tube just pulls through the P-trap nut. Am I missing something? I have what I think is a beveled washer that goes into the end of the discharge tube between the nut and the existing pipe (it is a solid white ring that slides into the discharge tube). I know I'm missing something...help is appreciated.