Help with Bugs

I recently downloaded the latest version of unity (4.0) (Thats my version anyway). This version comes with the built in game “Angry Bots” which is fine. To save myself some difficulty I started a new scene, rather than a new project, as you might expect. However, now, when I write some new code onto a model I created in Maya, there is an issue with the coding;

var LookAtTarget:Transform;

function update ()

{

transform.LookAt(LookAtTarget);

}

The Warning comes next to the top line and reads; ‘WARNING: Namespace “System.Collections” is never used. (BCW0016)’

However, that isn’t the main problem. The main problem is that when i run a debug a window pops up and says; “FATAL ERROR! It looks like another Unity instance is running with project open. Multiple Unity instances can’t open the same project. Project: /Users/Shared/Unity/4-0_AngryBots”

I have no idea how to get around this.

what are you computer specs

2011 iMac
3.1 Ghz i5 processor
8GB RAM
2TB HDD

Is that the problem?