System preferences, select "Dashboard & Expose"
There's a screen shot in a post above of what you should see. Look at the bottom where it says "Dashboard" and has F12 configured to bring it up. The other button is labeled as "---" because it's been deactivated.
If your middle mouse button is selected there, select the "---" option. You can still bring it up by F12 or hot corners.
I set mine up to display Expose when I move the mouse to the bottom left corner, and Dashboard in the bottom right corner. I used to have Dashboard in the Top left, but that was annoying. It came up whenever closing a window, or going to the "file" or "apple" menu in that corner.