When you click on the "Synths & Gear for Sale" section, " Marketplace Terms of Use" appears. In the upper right hand corner is "+ Pages: [1]" If you click on the "+," a menu drops down with "New Topic" at the top. If you click on "New Topic," a field for your post appears. That's the way it should work. None of this works for you?
I'm sure DSI can fix this once they're back from NAMM.