It is in active development. The development version can be retrieved on the subversion server at sourceforge. [link] When there is a binary release the sourcecode will be packaged up into a zip file too.
apoplugin.h is very complex. Big macros and ugly hacks The good news is it makes the actual plugin code much much smaller and easier to code. You don't need to understand everything that the header file does to use it. There is documentation on the wiki and examples on the svn server to get you started if you want to use it.