If you want to play a map of your design in Open Play, you do have to build it in the map editor. After that, follow these steps to play on it using Open play:

First off, make sure you have the game patch installed! You will need that to use monuments in Open play.

After that, all you do is this:

1. From the Open Play Options main window, look for the row that says 'City Map'. Just to the right of this, you will see a button that says 'Random City'. Click on this button.

2. A pop-up list of city names will appear. Scroll through the pop-up and click on the one you want to play. (This is where you would chose the map you had built, if you created one.)

3. You will then return to the main Open Play Options window, and the city name you selected will be shown instead of 'Random City'.

4. Do not put anything in the 'Random Seed' box.

5. Click on the 'To the City' button at the bottom right of the window. You will then begin play at the city you chose.