Mustee Durabase shower base installation

Hi! I've just purchased a 32X32" fiberglass shower base, manufactured by Mustee. The base has a built in drain, that connects to the pipe by a rubber seal.
I have some questions/concerns regarding this seal...has anybody ever used this product..? The directions indicate that you should just lubricate the rubber seal with liquid soap, then tamp it into the drain.
My concern is that this will not be a water tight seal, and will cause leaking. Is there an alternate/better way to fix this drain to the pipe..? If not...is there some kind of trick to getting the rubber seal installed..? I followed the directions, but I hammered away for nearly an hour, and still can't get the seal to go down flush with the top of the waste pipe.
Thank you!!

If all of the burrs aren't removed from the pipe and you don't lubricate the pieces, it can be a bear to insert the seal. It is more of a press/slide in place than pounding. Pounding, as you've found, doesn't really help much. My guess is that the pipe is not well supported underneath, and as you press, it is moving. You need to support the pipe, then you can press it down. If the pipe has a sharp edge and you didn't champher it a little, that will also make it harder.

Thank you for the suggestions...I will give them a try later this evening. If I can get that seal in as far as possible, will it seal adequately...or is there another preffered method to do this..? Would I need to add some silicone around the seal..?

Thanks for your help!!

-Heather