Data table error could not find function "."

Apparently the instructions I offered 2.5 years ago are still current for people using out-of-date versions of Mac R. Assuming you have Xcode and the Command Line Tools installed, you need to first either a) restart R (without loading any of the versions of data.table., reshape2, and dplyr), or b) remove any loaded Namespaces that might conflict with the ability to test-load the new packages:

unloadNamespace('data.table')
unloadNamespace('reshape2')
unloadNamespace('plyr')

Then build from source:

install.packages("data.table", type="source", dependencies=TRUE)

The reason that building from source might work when installing a binary package might not is that the former strategy gets you better checking for version dependencies.