use Nonlinear Curve Fit LM for Possion type fitting

Hi, guys,  
I am using <a name="parent" target="_blank">"Nonlinear Curve Fit LM</a>"
for a Poisson
type distribution. 




The Possion equation is f(x)=a^x*exp(-a)/ x!
Where x! is factorial of x.




How to write this equation to F(x,a) input?




Thanks for help!
Raymond