Howdy,
This forum's version of V-bulletin doesn't have the Quick Reply window, so you just skip the Go Advanced step. Advanced is already all there is. Sorry, I should have caught that for you.
You just click on Post Reply, then scroll down until you get the Additional Options window just below the composition window. The second outlined rectangle under the Miscellaneous Options block has the button called Manage Attachments. The rest of the instructions apply.