When you upload pics, do you use OCCs uploader? It should work the same.
In the correct video forum, find the button that says 'New Thread'. It should be near the top left. Click it and fill in the blanks. After you've named your new thread and written something about it in the big box, scroll down to where it says 'Additional Options'. Within that section you should find the buttons for attaching files. Either basic or advanced version, whichever you prefer. The rest should be self explanitory.
Just for your own information, sometimes the pop-up box for the downloader hides behind the window you're viewing. Depends on your browser and security settings.
Hope this helps! If it doesn't, let us know here, and someone will come up with an answer! OCC members are a pretty smart and helpful bunch!
