Automatic ppp ip address assignment

Dear All FreeRadius 2.1 users, I can run successfully with pptpd and freeradius 2.1 authentication very well. I need to assign ip address to each pptp user manually. However, I can't set up mysql ip pool for pptp user. I have tried this way: http://wiki.freeradius.org/Rlm_sqlippool but it doesn't work. ppp user cannot get ip address automatically. I also comment out main_pool inside the /site-available/default file. This doesn't work neither. I have also tried any possible combinations of other settings but they don't work. Any user would like to post the main setting of a successful automatic ppp ip address assignment through mysql ip pool? Your help is highly appreciated. Your Faithfully, Timmy