I can't figure out how to use the sneak attack ability. I have the arteial strick and all those feats but i dont really understand how to use them correctly??? Could someone please explain this?

You have to hide in the shadows then go behind an enemy and then you get a sneak attack, because you revealed your position and aren't hiding anymore.

i usually click on the weapon i am about to use untill the outline of the square turns red before sneak attacking

in 3E you dont have to hide in shadows. just flank your opponent and make sure he fights another one and your rogue will backstab with every attack.

Whats "flank" mean?

Flank means get behind.