random array variable

    public GameObject rndBall;

GameObject[] NBall = new USprefsPack.GameObject [6];
    BallList[0] = USprefsPack.greenBall;
    BallList[1] = USprefsPack.blueBall;
    BallList[2] = USprefsPack.redBall;
    BallList[3] = USprefsPack.whiteBall;
    BallList[4] = USprefsPack.yellowBall;
    BallList[5] = USprefsPack.purpleBall;

The question is: How to appropriate a variable rndBall any of values in array NBall

Are you intending to put the various colored balls into NBall, or BallList (which isn't defined in your snippet)?

Here's the simplest way to do it.

rndBall = NBall[Random.Range(0, NBall.Length-1)];

See also: http://answers.unity3d.com/questions/203/how-to-generate-a-random-number-inside-unity