Hello, I work for a landscape supply and design center (website not up yet, but whatsinyouryard.com) AND pond construction company (you can see some of the designs at lawnscapersgrounds.com) in NM.
Preformed pond "liners" are really difficult to work with and a big problem is getting your hole "good enough" to hold the preform in place. You're going to have to do a lot of back filling. We do not work with preforms, and we never will. Please don't be offended, they just look cheap, and are so darn hard to deal with. Plus if there is ever a hole it's a lot more time consuming to try to fix than regular rubber liner. I would recommend that you don't use the preform. Return it. But when you buy the flexible liner, don't buy anything less than 45 mil thickness. AND make sure to cover the bottom BEFORE THE LINER with either old carpet or thick landscape/erosion material that will help cut back on roots popping through your liner.
I see that you are in IL. I found a website that has some good info on it and the place in is Yorkville. Don't know how close you are to there, but here is their site.
http://www.pondsupplies.com/.
We buy all of our supplies from International Pond Supply (http://www.pondsource.com/) and would definitely recommend them. They can ship anywhere - FYI.
Hope this helps! Good luck with your pond building.
Wendy