The poissonpdf function calculates the probability at x for the discrete Poisson distribution with the specified mean u, which must be a real number >0. x can be an integer or a list of integers.
As an example, to compute the probability with mean 1.9 and x=4, press
2ND [DISTR] ALPHA [C]
enter 1.9 on the first line and press ENTER
enter 4 on the second line and press ENTER
press ENTER again.
This will paste poissonpdf(1.9,4) onto the command line. Press ENTER again to see the result (about 0.08).