I am not sure what you need to calculate. dpois() tells you the probability mass at a given number of counts. qpois() returns the number of counts below which a certain fraction of the probability lies. ppois() returns how much of the total probability lies at or below a given number of counts.
Compare the the plots generated by this code.
X <- seq(0,30)
P <- seq(0.05,0.95,0.05)
plot(X,dpois(X,13))
plot(X,ppois(X,13))
plot(P,qpois(P,13))