Each time you install an R package from the R command line, you are asked which CRAN mirror, or server, R should use. To set the repository and avoid having to specify this during every package installation, create the R startup command file .Rprofile in your home directory and add the following R code to it:
cat("Setting Seattle repository")
r = getOption("repos")
r["CRAN"] = "http://cran.fhcrc.org/"
options(repos = r)
rm(r)
This code snippet sets the R package repository to the Seattle CRAN mirror at the start of each R session