Once installed, add the following section to your trac.ini:
[flexjs] global = ext = local =
If you have any issues, create a new ticket.
Download the zipped source from here.
General instructions on installing Trac plugins can be found on the TracPlugins page.
If in your trac.ini file this configuration is added:
[flexjs] global = foo.js,bar.js local = blah.js ext = http://example.com/test1.js,http://example.com/test2.js
Will result in this addition to the <head> of your pages: