There is a Phone Booth Area to the right of the Food Court in the Mall. This has a phonebook with the name "Ghoulberg" in it, which is R.L. Stine's pseudonym. Choose to "Memorize" it in the phonebook, and a bubble will appear over the pay phone with the name and number. Then insert some Coins, which you get from the Fountain after you drain it, and call him up for your achievement.