Lubridate is actually an R package, not a function from R itself. You would need to include the jar for the lubridate package on your classpath.
Unfortunately, it appears that Renjin is not yet able to run this package: see http://packages.renjin.org/packages/lubridate.html for latest status of this and other packages.
Check back in a week or so for the next release candidate, we're fixing a lot of these problems.