I just wanted to add that I recently had this problem - it seemed like somehow the program had gotten its default link values mixed up - everything just went to the main store page. I think a few things contributed: one, my client had a lot of "store" pages left over from at least one other attempt to install a shopping cart, and because of the word "store" in the slugs it kept sometimes using those pages. Also, I think some of the pages it *was* supposed to be using became unrelated as child pages to the main store page for some reason--when I looked at pages like Cart and Receipt they didn't show any parent page at all (I could have just reassigned them, but I had already decided to start over and be thorough). Thirdly I'd changed the name of the store page several times. Somewhere along the line, the default values of things had gotten mixed up.
I looked in the database via phpMyadmin to see if I could change the values by hand, but it was late and I wasn't feeling well and couldn't find it. So later I just exported the "wp_simpleecommcart_products" table and a few others from the database, deleted the plugin, deleted all the pages it had created plus the ones that were causing confusion (and removed them from the trash, too). I reinstalled, uploaded the tables again and everything worked.
Just recording my steps in case someone else has the same problem.