Installing R Packages on ubuntus

Hello,

 command line so as to run R. Inside R, I typed >
 install.packages("raster",repos="http://R-Forge.R-project.org<http://r-forge.r-project.org/>")
 so as to install the package, but no luck. Below is the error message:

If you're running an amd64 Ubuntu Jackalope (something you didn't
quite specify), the following might work. Add
deb http://debian.cran.r-project.org/cran2deb/debian-amd64/ testing/

in
/etc/apt/sources.list

These binaries are built for Debian testing, but they seem to be
compatible with Jackalope. Reload Synaptic, and install
r-cran-<whatever>.
Liviu