RSS-feed with <Pubdate> tag

I am looking for a way to get the tag <Pubdate> in to my RSS-feed.

I first found this mod by Alex King, but I don't know how to install it:

http://punbb.org/forums/viewtopic.php?id=6586

Could anybody give me a hint on how to install it?

Otherwise I have been looking in the file extern.php, and to me it seems like I only need to add an ekstra line for outputting <Pubdate>. Any proposals for how that line has to bee?