You just need to install addin4githubinstall package (plus its requisite dependencies) with:
devtools::install_github("gepuro/addin4githubinstall")
After installing this package, you should see this addins on toolbar menu.
If you can't not find this addin on Addins menu, you may have to load this package explicitly on RStudio console:
library("addin4githubinstall")
Type the package name which you want to search in Search in Title box.
After selecting the packages, push Install button.
