To start with you go to C\Program Files\Ubisoft\SH3 and you double click on the file called Mission-Editor.exe. That will open up the mission editor. After that you open the Campaign_SCR.mis located at C\Program Files\Ubisoft\SH3\Data\Campaigns\Campaign. That will bring up the world map with all the SCR data for the campaign. Then you chose the option Time filter start on the top up title bar and you set the starting time to 1939 and the month to 10. After that you zoom in close to England and you see the minefields.
As you can see in this screenshot they're quite large and extended, so be very careful
EDIT: To navigate around the map you need to click on the little hand icon on the top task-bar
EDIT II: You're correct BBW. I've croped my picture a bit. Hopefully it's better now