For general instructions, see: http://cran.r-project.org/doc/manuals/R-admin.html#Managing-libraries.
In Ubuntu Linux, the path to the global libraries is: /usr/local/lib/R/site-library/
To install there, you can do install.packages(c('foo'), '/usr/local/lib/R/site-library/')
or take advantage of the built in variable: install.packages(c('foo'), .Library.site)
Check that .Library.site has the values you need.
You can also use R CMD INSTALL -l /path/to/library foo