The map editor is in Door placement mode.

F2 switch to picture mode
F3 switch to passability mode
F4 switch to door placement mode
F5 switch to NPC placement mode
F6 switch to foemap mode
F7 switch to zonemap mode

Use the arrow keys to move around the map.

Use SHIFT+arrow keys to move around the map faster.

Use ALT+arrow keys to move the view of the map.

Press the SPACE bar to place a door.

If you want to remove a door, position the cursor over it and press SPACE again.

To link a door to another door, position the cursor over the door and press ENTER. This will take you to the door linking tool.

Remember that for a door to be usable in both directions, it needs to be linked from both directions. If you only link a door one way, it will only be usable one way.

CTRL+S immediately saves, useful when live-previewing a game.

TAB toggles a blue rectangle that shows roughly what area of the map you are currently looking at.