Sorry, but you are learning a valuable lesson through all of this. Unfortunately there are some LFS out there whose sole intent is to make money and they will tell you whatever they need to in order to sell products. There are many stores that actually care about the animals and what it takes to properly care for them, but it sometimes takes a little while to find them, especially when you are first starting out and do not know what you need.
How much of the calcium additive did you put in the tank? Chances are it wasn't enough to cause any serious problems right now. However, if you continued to dose with it then it would continue building higher levels. I would not use it anymore until you can get a test kit that will test your calcium and alkalinity, aka dkh.
Unfortunately I am not familiar with your system so I need to ask some questions to figure out where you stand right now.
What size tank is this? Larger tanks can dilute things much better and the old saying goes "dilution is the answer to pollution".
What inhabitants are in the tank? If you do not have any sps corals or clams then there is a chance that you will never need to dose calcium. This is because your system may maintain adequate levels just from normal water changes.
Of course, the only way to be sure is to use a test kit and figure out where your water currently sits. To learn more about the interactions of these items in your acquarium you may want to read this article:
http://www.advancedaquarist.com/issues/nov2002/chem.htm It is an oldy but a goody.