I have a feeling you'll have to modify the question to make this answer invalid, but the natural answer is to induce people to have more offspring.
In your question you state the goal of the society is to "maximize total pleasure." Total pleasure is most easily maximized by simply maximizing the number of individuals, rather than maximizing individual happiness. Thus, the easiest solution is to simply have everyone reproduce like rabbits, maximizing the number of individuals that can be happy. This is a known issue in creating naive utilitarian societies whose value metric is the sum total of everyone's happiness. There's also several other issues. Maximizing happiness turns out to be something we've spent a long time thinking about and yet still aren't very good at!
Of course, if you ignore the philosophical side of things, and focus on chemistry, the story is a bit harder. The concept of pleasure is noticeably difficult to pin down. Every time we think we fully understand it, something else changes our points of view. One of the real challenges of such a chemical change is that the society changes when the chemical is introduced. If everyone is suddenly in extacy and needs no further outside stimulus, it's highly likely that everyone will starve. If you rely on machines to feed them, eventually the machines will break down (and we have to start playing with the philosophical questions about whether the machines are happy).
On the short term, electrical stimulation of neurons has a track record of being effective. There have been studies which show it possible to induce everything from euphoria to orgasm using electrical stimulation in the spine (the study I remember reading about was actually intended to explore a different side of electrical stimulation for rehabilitative purposes, and discovered this effect by accident). However, in the long term, the brain is an amazingly capable balancing engine. It eventually tunes such simple stimuli out.
On the long term, my recommendation would be to take lessons from those groups that claim to have the secret of eternal happiness. Many groups, from religions like Buddhism, to spiritual groups such as those of yoga, claim that if you practice their art for long enough, you can achieve a continuous state of bliss. Interestingly enough, all of them suggest a very subtle bliss -- not something over the top. Perhaps thats the place to start.