Thread Subject: problem with curve fitting

Hi everybody,
I'm trying to fit a curve to my data using 'curve fitting toolbox', but I can't get any result from.
here is my data:
x=[0 2 50 106 250 500 1000 2000];
y=[0 0.03 0.17 0.279 0.735 0.921 0.994 1.002];

and here is my 4-parameters function:
y=(1/log(exp(1)+(ag/x)^ng)^mg)*(1-(log(1+(dg)/x)/log(1+dg)))^7

One of my friends has done this job using the SPSS software, and I know the values of parameters are:
ag=250 ; ng=2.944 ; mg=1.2 ; dg=3.059 ;

I've tried different boundaries and starting point! where am I wrong or It's something whit this toolbox?
(I'm using v. R2007a)

thanks in advance,