What is the name of the package for R?

This is merely a tip: Open synaptic, and search for r-cran or just r-, there's a r-recommended GNU R collection of recommended packages [metapackage] I believe this is the one you're looking for.

The name of the package for R is r-base-core.

Whenever you know the exact name of a program that is not installed, you can just try to type it in the terminal as if it was installed. The terminal will give you feedback if it knows of a package that has the program that you typed in.

This is what you get when you type R at the command line:

The program 'R' is currently not installed. You can install it by typing: sudo apt-get install r-base-core

In this case, you can just copy the command into the terminal and just run it.
Sometimes the terminal will tell you this if you just mistyped something, so you can ignore it in those cases.

I just installed the package, just to make sure. It seems to have required 96 MB of my hard drive.