You're looking on the product download page, right?
http://www.abisource.com/free.phtml
Just below the list of platforms, you should see a paragraph which reads as
follows:
"Since AbiWord is Open Source, you always have the option of downloading
the source code for AbiWord, modifying that source as much or as little as
you would like, and then building your own unique personal version of
AbiWord. For directions on how to do this, see the Developer section of
this site."
Click on the word developer. :-)
Or, you can always get there from the "developer download" link on the
sidebar, which links directly to the appropriate page:
http://www.abisource.com/dev_download.phtml
Enjoy!
Paul