In the MODS section, click Single Player. Select Set Up Game. The Map should be selectable here.
If you included a Scenario with it, you'll need all applicable mods to also be enabled. If they are, the Load Scenario button will appear in the Map Type area of the Set Up screen after you've selected your map.
If any of this isn't working quite right; that is, your map isn't in the list in the Mods Single Player area or you don't get the Load Scenario button with everything you need enabled, then there is something not quite right that needs to be addressed... look through some of the forum threads on such topics to find if anything matches your experience.