Trying to run Rcpp in R but unable to create makevars file

Question

There have previous discussions regarding the compiler in XCode and its relation to the Rcpp package. For instance, here https://stackoverflow.com/a/19550947/1786009 It seems that the answer is to create a Makevars file in the directory ~/.R/Makevars My question is simple, how do I find where the .R directory is?