In order to get to Bean Valley, you'll have to scale a cliff. This cliff is just beyond the last Armored Ant (that hides in swirling sand) you saw before entering Belome's Temple (think 'top left-hand corner of the screen'). Before rushing off to the cliff, remember to talk to the Monster Mama (you'll find her inside the first 'house' of Monstro Town if you are coming into town via the Belome Temple). She'll start to brag about the town's 'star'. Go upstairs and talk to the pink star - it will do something seemingly unimportant - then go talk to the Monster Mama again. Once she discovers that you are looking for a star piece, she'll ask a bunch of sky troopas to help you scale the cliff. After all of this madness occurs, you should be able to scale the cliff to get to Bean Valley with the help of these sky troopas. I'm not sure I've ever seen a 'Bean Town' in Super Mario RPG, so I hope I was correct when I assumed that you were referring to Bean Valley.