I'm begginer in electrical & electronic subjects,so I didn't understand when you say "you have to have synch wih the AC mains.".
That's ONLY required for dimming. If you are not dimming, don't worry about that.
If you can't turn the light on/off with 5V, you may have fried the opto-coupler because you didn't have a resistor. It's also possible that you fried your Arduino, so just check the opto-coupler/TRIAC with 5V first (no Arduino). When that works, try it with the Arduino.
I'm begginer in electrical & electronic subjects
Since you're (apparently) not dimming, a MUCH SIMPLER alternative is a [u]Solid State Relay[/u]. (You are building a solid-state relay with the opto-coupler and TRIAC.... But your's isn't working... 
)