Pajek is powerful, free social network analysis software. It runs natively on Windows, and via Wine on Mac or Linux.
To download and install Pajek, follow instructions on the Pajek wiki. For compatibility with the book, download the ESNA Book Edition version (October 1, 2004), named "Pajek.be.exe". You may like to download the latest version as well, but without the older Book Edition there is no guarantee that everything will work as the book says it should. On the other hand, more recent versions may be improved, and will provide basically the same functionality. Your pick. You might like to have both.
Special instructions for the Mac: Installation on a Mac requires that you first download and install WineBottler. Ensure that you have X11 (supplied by Apple) installed as well. Download Pajek.exe and open it with Wine. You will then be asked whether you wish to run using Wine, or to create a standalone application using WineBottler. Note that Macs require the 32 bit version - 64 bit version will not work (at least not on my mac).
I suggest you select the latter option, which will create a standalone Pajek application for your Mac.
Note: if the above site is down try downloading the WineBottler installer from here.
Data files required for ESNAP (and manual)
Download these sample data sets for use with the textbook. Store them in your Pajek Data directory for future reference. You'll also find a Pajek manual for reference (but ESNAP is a better guide).
Other software for Pajek
These two programs are very useful for generating Pajek files out of data presented in a different format:
txt2pajek: Converting text file data sets into Pajek format
excel2Pajek: Converting excel sheets into Pajek format
More help is available!