Click the manage attachments button. A popup window will appear.
Click browse under Upload File from your Computer. and select the file you want to upload.
Then click the upload button.
After the file is uploaded it should show the file name to confirm it has uploaded.
Then click close this window at the bottom of the page.
If your upload worked correctly then the filename should also show up under additional options in the attach files area (after you close the popup window). If it is not there something went wrong. Could be a popup blocker or ad blocking software or something else that is interfering maybe?