1) Click the "my garage" link just under the "m" in the Motortopia logo, and scroll down. You should see a box in the middle column of your garage page entitled "Parts" and then you should see an "add part" button. Click that to add a part for sale or part wanted.
2) Click "parts" in the red main navigation bar at the top of the page. The "parts" link shows up just below the "cars" tab. There will be a button to click there to add a part. Funny thing is, the "add a part" button on the "parts" home page is on the developement version of Motortopia.com, but I completely forgot to make the button show up on the live site that everyone visits, of course.
Anyhow, the "add a part" button on the "parts" home page will appear in a few minutes.