Installing an R package from local unzipped folder

The solution to installing a package that's been unzipped into a folder is as follows:

install.packages("C:/path to folder with the package", 
repos = NULL, 
type = "source")

I think the error message is actually just wrong. You need to give the file name of the package, not just the directory.

install.packages("C://path//to//directory//MY_PACKAGE.tar.gz", 
    repos = NULL, 
    lib = "C://path//to//newDirectory")

If it is an unzipped Windows binary (e.g., from CRAN), you can just copy and paste the entire package directory into your library folder. You could also, presumably, use file.copy() to do so if you wanted to do it within R. install.packages() is failing (weirdly) because you're giving it something other than the typical package source or zipped binary that it is expecting.