Hi Ben,
we had the same error. It is easy to fix, because i think you didn't follow up the whole installation instruction - STEP 10.
I stopped there first too, because i thought it would be optional an i didn't want to change the tep_get_products_special_price ...
But if you read carefully you will notice that you should copy your actual tep_get_products_special_price from the catalog to the general.php in the admin section and that there is no need to change your own code - just add the function.
Then it worked fine.
Hope this helps