The excretory system, used to remove toxic byproducts of digestion and metabolization as well as tissue breakdown, is located on the American lobster below the antennae. Urine is released through the same area, in the excretory organs, however can also be released through the gill and when the lobster molts. Often times urine is released out of pores in the shell when it does't need a significant amount of purification to extract nutrients, or simply isn't beneficial to the Lobster, a prime example of this would be salt water.
Left: Poisonous fluids that, if left in the body, will become quite harmful and destroy the lobsters kidneys. The most significant of this is urea, the result of broken down amino acids.