From MEPIS Documentation Wiki
R is a powerful open-source programming language and software environment for statistical computing and graphics. It can be extended through community packages covering specialized areas such as statistical techniques, programming interfaces and import/export formats.
R is available in the repos for MEPIS 6.5, 7, 8, 8.5 and 11. Using your favorite method, install
which will bring a number of other packages along with it. For more recent versions, check the R homepage below under Links. At your discretion you may also want to install the help files - r-base-html, r-base-latex, r-doc-html, r-doc-pdf and r-doc-info. Unless you're really tight for disk space there isn't much reason not to have these. You will also see a number of packages from the CRAN repositories listed here, which you can install as needed.
A good GUI exists for MEPIS called RKWard that is integrated with KDE and is particularly useful for beginners. It exists in the repos, although that version may be well behind the current version. Install
Once that and its dependencies are installed, download the most recent Dapper (for MEPIS 6.5) or Etch (for MEPIS 7: rkward_0.4.9-2_i386_etch) deb package from this page and install in the usual manner.
If you prefer using a CLI, which is the original and more rigorous method, then you will need to learn the language commands. A helpful place to begin is this short reference card, and there is a wealth of information on the R Wiki linked below.