Julie Koesmarno made a great post on installing R packages. Please follow this post. Also Microsoft suggests the following way to install R packages on MSDN.
Since I wanted to be able to have packages installed directly from SQL Server Management Studio (SSMS) here is yet another way to do it. I have used xp_cmdshell to install any additional package for my R (optionally you can setEXECUTE AS USER).
This is a bit of a backdoor method, but it does work.