To blog, here's what you do:
First, make sure you have a working copy of your project's CMS Repository Location
place your file in the blog directory within your projects working copy - typically <portal-name>/members/<project-name>/blog/
the file can be any text file; its content will be displayed as the blog text
set the "title" property of that file (using svn properties). Under linux, type: svn propedit title <file-name>. If you use tortoise SVN, this is even easier - just go to the appropriate tab.
commit the file - it should be visible shortly afterwards
If you want to have a "Blog" page displaying a non-standard feed, you need to add the following lines to your project.xml (the menu entry must be there regardless if you have a standard feed or not):
<feed-address>http://labs.jboss.com/feeds/<project-name>/blog/rss2</feed-address> <menu> (...) <page display="Blog">blog</page> (...) </menu>
In fact, if you have an outside blog, you can display it this way too. Just set the appropriate address.