The first input to ezplot must e a function handle or a string (see http://www.mathworks.com/help/matlab/ref/ezplot.html for details). Your variable d is neither - are you using the Symbolic Math Toolbox with a previous command like sym x in order to get that command to evaluate successfully?
I was able to plot this function as follows
d = 'x.^2 - 6*x - 12'
ezplot(d)
or as
func = @(x)x.^2 - 6*x - 12
ezplot(func)
Note the addition of the period at x.^2 so that func can evaluate an input vector rather than just single elements.
