Based on the above information I think while calling the errorbar function you have to first compute the value of y coordinates from the fitted curve and then call the errorbar funtion on these new values in order to get the errorbar on the curve.
load franke
cfit = fit(x,y,'poly3','normalize','on');
figure
plot(cfit,x,y)
hold on
yfit = cfit(x);
err = abs(y-yfit);
errorbar(x,yfit,err,'go')
If this is not what you are looking for, can you provide more information by attaching images of what should be the output plot and any other required information.
