You can install the stable version on CRAN:
install.packages('np', dependencies = TRUE)
library(devtools); install_github('R-Package-np', 'JeffreyRacine')
Note Windows users have to first install Rtools, while OS X users have to first install Xcode and the command line tools (in OS X 10.9 or higher, once you have Xcode installed, open a terminal and run xcode-select --install).
Note that if you wish to install the MPI-enabled development version of the package (i.e. the package `npRmpi'), you can add the option ref='npRmpi' to the install_github call above presuming that your system has the required MPI subsystem installed (see my homepage for further details).
For more information on this project please visit the maintainer's website (http://www.economics.mcmaster.ca/faculty/racinej).