Where you post the thread you are writing, there are 2 buttons. Quick reply(?) And Go Advanced.
Click Go Advance and to the right of the Smiley you will see a Paper Clip and this is where you attach a file, for example the log. When you have clicked the Paper Clip there will be a pop-up where you can select the file from your harddrive and then select Upload that is located to the right.
Screenshot where Paper Clip are.