put the file in the "public" folder...let it synchronize...indicated by a light blue circle thingy under the file...once that is done...right click the file...then find the word "dropbox" in the drop down menu...hover your mouse over it and locate "copy public link" and click on that.
then in your forum post, right click in the message box and click on the word "paste"...a url will appear...put the "url" brackets around it, and it will be a live link that all you have to do is click and it will prompt you to save or open.