Help with nozzle!

I'm working with a Prusa Mendel with an Arduino board and "repsnapper" software. All motors are moving and the bed heats. The main problem is that the nozzle doesn't heats.

I've no idea of electronics but I'm sure that I've no problems with the connections. I mean, when I connect the positive wire in his own place, in one hole of D10, and I put the other one in ground it heats. But doesn't works if both are in D10 holes!

So I don't know if the software doesn't give the orders or if the board doesn't do what it needs to start heating... or what? I'd like to know if someone had the same problem or if knows which part of Arduino is exactly the one which has to do that.

Thanks a lot!

Done!

What was your problem cause and the solution ?

I was wrong and, in my motherboard, I need to connect the nozzle in D9, not in D10 as I did...!

Thanks.