This is how I installed it, its ugly for now, but it works.

R CMD INSTALL ./rmemcached

You might need to provide the path to libmemcached like so:

R CMD INSTALL ./rmemcached --configure-args=--with-libmemcached-path=/usr/local/

library(rmemcached)
server <- cache.connect("127.0.0.1", 11211)
cache.set(server, "foo", "1000")
cache.set(server, "bar", "2000")
cache.get(server, "foo")
cache.get(server, "baz")