Be careful when using this argument. It can be stated as following:
1. If an omnipotent and omnibenevolent god exists, then evil cannot exist.
2. There is evil in the world.
3. Therefore, an omnipotent and omnibenevolent god does not exist.
Believe it or not, it is possible to screw up this argument - it is not all powerful, like some young atheists think. The theist can argue that it doesn't make sense for god to stop ALL evil, as he would then interfere with the free will.
So, Theist defense against it is that even if logically god could make both free will and that people always choose good, that world would be inferior to the world where people really freely choose between good and evil. Of course, you might get into a whole discussion what free will even means, etc ... but, why complicate it?
You can also argue that god is at fault for allowing evil to exist in the first place or that he's partially responsible for suffering because he could have known the consequences of creating beings with free will, but again ... why complicate it?
Better not to use this argument 'as-is', as a point of logic. Better use it to respond to the claims how god has a plan, etc etc, for example, he would help you find your car keys or score a goal, but he will not prevent rape. God does interfere with the world by making one team win, but such things are argued only by the most literalistic theists.
Of course, the argument is much more powerful when it comes to NATURAL disasters, or birth defects, or something which doesn't have anything to do with free will, because god preventing natural disasters cannot have any effect on free will. But, you will notice, the most successful version of is is not used as a point of logic - meaning it does not want to disprove god, but to show that the existence of evil makes god not impossible, but unlikely.
This is called Evidential problem of evil and it can be stated something like this:
1. There exists suffering which does not contribute to any greater good, so God could have prevented them
2. An Omnibenevolent God would prevent those evils.
3. Therefore, it is not likely that an Omnibenevolent God does exist.
You can see that this argument cannot be so easily dispelled or robbed of power by simply invoking free will, because it requires very little setup - only the fact that SOME suffering cannot or does not lead to any greater good and it does not interfer with a free will, so an omnibenevolent God would want to prevent those occurences of evil.
Clearly, there is great potential to misuse Epicurus' argument and you should be careful when doing it. There are other ways to approach the issue, if you really want to argue that god doesn't exist.
But, again, an atheist doesn't need to show that god cannot or doesn't exist. Being an atheist simply means you did not find theists' arguments FOR god convincing. That is it.