The netZ package is stored at
http://www3.imperial.ac.uk/theoreticalsystemsbiology/data-software

To install netZ, copy the tgz file into a suitable directory and call it netZ.tgz. Then run:
R CMD INSTALL netZ.tgz

You can call it from within R using
library(netZ)  

For further details on the R statistical environment please consult the R-project webpages. You will also need the 'gsl' package which can be downloaded from CRAN.