if you open you R and do the following, do you get an error?
library(ggplot2)
dat <- data.frame(x=rnorm(10), y = rnorm(10))
ggplot(dat, aes(x=x, y= y))+geom_point()
If it does, then you will need to check if the ggplot2 installation, and the graphic backend of your R installation match.
Sam