Did you install the package using the R client, on the machine in question? Also, have you added the location of the R software to your SYSTEM directory (Environment variable)? I had the same issues with all packages and after adding the folder to my SYSTEM directory most everything was cleared up. Anything that wasn't was due to it not being installed in R itself. Not sure if this is the same issue you're experiencing, but it's worth a try.
sehunley
sehunley