Before installing any R packages, be sure that you have already configured R for use with the DSS. DSS does not embed its own R environment. Instead, it relies on the host R's installation.
Assuming you have sufficient privileges to do so, there are several ways to install new R packages:
mbp:~ thomas$ R
install.packages( 'data.table', repos = 'http://your-favorite-cran-repo.org', lib = '/path/to/your/writable-directory-for/R-packages' )