When you reply or create a new post go to the bottom of the page and click on the "File Manager" link. That brings up a window that has a Browse button to browse to the file on your computer. Then click on "Add File" ... and wait a while until it uploads ( there is actually a small progress bar ). And it will take a little while. When it has finished the file name will appear at the very top of the window ( don't mistake it for the file name that appears when you Browse to it ).

Then click on "Done Adding Files" and it will appear in your posting.
There is a limit of 2 files per post.