- There are many ways of doing so but I prefer that you should get a window with a list of repositories or a text menu with some options. 
- But sometimes that doesn't appear in that case, you can always specify the mirror from where to download the packages yourself by using repos parameter. 
- Doing that, R will not about the repository. 
- For Example:
                                   install.packages('RMySQL', repos='http://cran.us.r-project.org')