What version of R are you using?
Also just to double check, the iris data is loaded into memory right? Try typing iris in your R Console.
If it throws an error, then try loading it into memory:
data(iris)
If iris was loaded into memory, and the error still persists, then try creating a simple bar plot:
dat <- data.frame(cond = factor(rep(c("A","B"), each=200)),
rating = c(rnorm(200),rnorm(200, mean=.8)))
library(ggplot2)
ggplot(dat, aes(x=rating)) + geom_histogram(binwidth=.5)
And see if ggplot throws any errors.
Kind regards.