Well, to post a clip, just follow these steps. Let's just say you are starting a new thread:
1. Click on the "Post New Thread" button. (Which you must have found, since you created this. ^_^)
2. Scroll down the screen, and below where you type, there is an area labeled options with several check boxes. Look just below it.
3. The area labeled "Attach File" is located there, and in it there is a button labeled "Browse."
4. Click that, and browse through your computer's files to find the clip you are looking for.
5. Select it and click "Open," located in that same browsing window.
6. Then, submit new thread!
Note: You cannot upload any file larger than 5 MB.
I hope that helps and if not, let me know and I can walk you through it in chat.