Non-visible functions: merge.data.table

I've downloaded the "data.table" package from CRAN and R-Forge and still
can't utilize merge.data.table for faster merges. How do I make this
function visible?

install.packages("data.table",repos="http://R-Forge.R-project.org")

trying URL
'http://R-Forge.R-project.org/src/contrib/data.table_1.5.1.tar.gz'
Content type 'application/x-gzip' length 616492 bytes (602 Kb)
opened URL

library(data.table)
methods(merge)

[1] merge.data.frame  merge.data.table* merge.default

   Non-visible functions are asterisked

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