You've just posted a filepath...guessing you were trying to upload an attachment. On the post page, scroll to the bottom and click 'Manage Attachments' then upload from there.
Also have a look at the following links, as the information presented there might be helpful in your future endeavours into Mac OS X and could clear up initial confusion and may even prevent harm to your system or your files.
Mac OS X Basics
Switch 101 - guide with articles made by Apple on how to accustom yourself, after you switched to Mac OS X from Windows