I'm with Ding 100% on that, except that if you exit the front entrance and trun right for the bus stop it is West of the Mall (a minor detail).
This is one of the easiest bus trips you can take in SJ, as just about any bus heading in that direction will take you downtown for about $.25 and one leaves for the 5-10 minute trip every few minutes. The main downtown bus stop is across from the Costa del Sol (1 block east of the Prez) You'll know when to get off, cause everyone left on the bus will most likely be getting off there too.
Heading OUT to the Mall is a little bit more difficult though still not hard. I'm not sure if all buses that stop at that corner will be heading back to San Pedro? I usually go down one more block and cross Avenida 2 to catch a bus going in the right direction. Again, unfortunately not all buses will be going by the Mall, but a lot of them will. Just look for a bus with a San Pedro sign in the front window or even ask someone waiting at the stop if you're waiting at the right place (no spanish necessito, a simple "San Pedro
" works just fine). You can ask someone on the bus where to get off for the Mall or if the bus passes under the autopista you'll know you went to far. BTW, my new map has a picture of the Mall building so you'll know how to spot it even if you've never been there.