To start off , you have to be in the Discussion itself. You can't attach files when replying to a Discussion from your Inbox(thank you Dan Pihlaja for pointing that out). Also, if you're attaching a Drawing or Assembly, please keep in mind that you also have to attach all referenced files or we won't be able to open it. Please don't post these files separately, but do a Pack and Go to a zip file and post that (see How can I create a new Assembly or Drawing similar to an existing one? if you aren't familiar with Pack and Go).
Now that I have that out of the way, if this is in a Reply to an existing Discussion, click on the black text "Use advanced editor" in the right corner above the text box. I know it doesn't look like a link, but it is.
After clicking on that you'll get a link to attach files in the lower right, beside the "Mention" link shown above. Click on the "Attach" link and Browse to and select the file.
If instead of a Reply this is a Discussion you're just starting, the "Attach" link should already be there. If you've already posted the Discussion and would like to go back and add a file to the original post there's an "Edit" link under "Actions" to the right of your original post you can click on to get the "Attach" link back.