At the top of each Thread choose the option - Thread Tools - Subscribe to This Thread - Then for email notifcation, you can select the desired option. Usually instant notification is best, since you want to know when someone has post in this thread
I dont think you can do it on wap...gotta log in to the regular site. One option is to log in to the regular site thru ur BB browser/Opera Mini and then subscribe to the thread. It'll take longer than the wap website but at least you'll be able to subscribe and then resume browsing the wap site.