You have to order it online from companies in France. The stuff they sell in ABC stores isn't really absinthe.
http://www.absinthebuyersguide.com/
You have to serve it right, and even then, the effect is almost imperceptible. At least it was for me, but I expiremented with the standard fare of recreational drugs in College, so maybe I had higher expectations. Shrooms and Weed have distinct effects. Absinthe didn't have a very distinct effect.
You also have to pour cold water over a sugar cube on a spoon over the absinthe. Something about the sugar helps to increase your body's absorption of the wormwood oils or something.