To set up a single player mission you are going to need a player character(PC) to control and also enemies or other non-player characters(NPC's) to interact with. Here is how to set them up:
1. The first step is to click on the "Level" button at the top of the level editor. On the left side screen there should now be an option button for "Player" - click it. This opens the "Player Setup" dialogue box. The default settings are "0. Scenery" and "1. Humans". You want to "Add" another Player Index - "2. Enemy". Under "Name", type "Enemy" and for type you want to use Computer. The team you can pick "2" and then click ok.
2. Now click on the "Team" button on the left side which will bring you to the "Team Setup". Highlight Team 1 and type in the name of your team - "BOS" or whatever and the same for Team 2 - "Enemy". Where all the green squares are, left-click the appropriate boxes to make them enemies or right-click them to make them friends. So in the first row, left click on the box in column 2 until you get a value of -10 and the same for Row 2, Column 1.
3. Now you want to insert the entities for your enemies. At the top of the screen click on the "Entity" box. A new menu will appear on the left side with a directory of the entities. In the Player field enter the number 2 and hit enter. Don't enter anything in the Tag field for now. Click on the directory for Actors and choose the character you want to use or find an entity that you have already created with the entity editor. Then just place him/her wherever you'd like. If you immediately click the entity editor box you can edit this entity. You may want to customize the looks of a generic figure or rotate him to face a particular direction or add things to his inventory.
4. If you want to add a PC that you control, do the same thing except put him/her/it under Player Index 1.