I agree with Ddlv, a series would be better.

I'd assumed on a quick look that you had used series, so my references to ITrend[1] not existing is of course only true if it was series like Price but not for the array.

Code:
vars Price	    	= series(price());
vars ITrendline 	= series();

if (Bar < 4)
{
	ITrendline[0] = Price[0];
}

else
{
	ITrendline[0] = (((Alfa - ((Alfa /2) * (Alfa/2))) * Price[0])
					  + (((Alfa * Alfa)/2) * Price[1])
					  - ((Alfa - ((3 * (Alfa * Alfa)) / 4)) * Price[2])
					  + ((2*(1-Alfa)) * ITrendline[1])
					  - (((1-Alfa) * (1-Alfa)) * ITrendline[2]));
}


Seems to work.