Every time you install a R package, you are asked which repository R should use.
To set the repository and avoid having to specify this at every package install, create a file .Rprofile in your home directory with the following text:

       myrepo = getOption(“repos”)
       myrepo[“CRAN”] = “http://stat.ethz.ch/CRAN/“
       options(repos = myrepo)
       rm(myrepo)