The models are currently provided on a chapter by
chapter basis, and are archived using the 'Stuffit' format.
This compression/archiving format is common among Macintosh users, but will
often require downloading special software for PC/Windows systems, where
it is less standard.
Aladdin Systems' Expander is one such freeware product for PC/Windows. Download it from http://www.aladdinsys.com/expander/expander_win_login.html
Note that Expander is a very handy product to have installed on your PC in any case, for unzipping, untarring and unstuffing archives.