You almost can't miss this Achievement, and you don't have much control over it anyway. (<-- not so sure about that anymore) When you're playing the Barcelona game, customers will ask you to bring them shells. Pick up the appropriate shell and bring it to them.
Eventually, you will be asked to retrieve each different kind of shell at least once. There are three different kinds. Once you deliver the third type of shell, you'll unlock the Achievement; you don't need to complete the game to earn the Achievement.
EDIT (AGAIN): It seems like you might need to deliver the three unique shells in a row, without delivering drinks in between. In the comments, M0NKEY R4NGER provides the best way to do this: Wait until two guests request two different shells. Deliver the shells, and then don't deliver anything else until a guest requests the third type of shell. Obviously, there's a little luck involved. Moving guests between the chairs and the pools is okay. Just don't deliver drinks or duplicate shells.
EDIT: It seems like some people are having trouble unlocking this. It is possible that you may need to deliver each of the three different shells in a row (as opposed to, for example, two sand dollars, three starfish, and then the other shell). However, that is not confirmed. I wasn't really looking out for that when I earned the Achievement. I'll update the solution if anyone else is able to confirm one way or another. Of course, it's also possible that the Achievement is buggy, which in the worst case, you might need to uninstall the game and reinstall it and work you way back up to Barcelona.
FWIW, U2NUMB says,
"Mine was glitched for a while but after my 5th time trying this it unlocked. I did them in this order.. Sand Dollar, Shell, Star"
So if it doesn't unlock at first, keep trying.