Side Scrolling Shooter. How to aim my bullet trajectory at the mouse cursor?

I have to say, I didn't download the project. But from the code you posted, something along these lines should do the job. The trick is - to read the current distance from the camera (to the player's ship, or whatever is launching the projectile). Then convert the mouse position to a world position at the same distance away from the camera.

You can then use the normalized difference between these two points as the velocity - multiplied by whatever speed you like.

(untested code - for illustrative purposes, may contain errors, etc!)

var projectile: Rigidbody; var projectileSpeed = 20; function Update () { if(Input.GetMouseButton (0) ) { var instantiatedProjectile : Rigidbody = Instantiate(projectile,transform.position, transform.rotation);

     // get current distance from camera
     var distFromCam = Camera.main.transform.InverseTransformPosition(transform).z;

     // get target in world space, at same distance from camera:
     var targetScreenPos = Input.mousePosition;
     targetScreenPos.z = distFromCam;
     var targetPosition = Camera.main.ScreenToWorldPoint(targetScreenPos);

     // calculate direction & velocity:
     var targetDelta = (transform.position - targetPosition);
     var launchVelocity = targetDelta.normalized * projectileSpeed;

     instantiatedProjectile.velocity = launchVelocity;
 }

(If all your objects are always at the same fixed distance away from the camera, you could optimise this code by having that as a constant value rather than looked-up using InverseTransformPosition)