Is there more to the error than that? For instance, that line is usually followed by a line saying something like

there is no package called ‘x’

or

namespace 'x' is already loaded, but >= x.x.x is required

If the error is not solved with the solution Yurii provided, you can try this to install ggplot2:

install.packages( "ggplot2", repos = c("http://rstudio.org/_packages", "http://cran.rstudio.com") )

Hi,

My recommendation is:

1) install the most up-to-date version of R

2)install ggplot2 as #Ole Goltermann has mentioned

Good luck,

Hamid