Every map needs three things in order to compile and play properly. They are:
A sealed-off world for the player to move around
A player start entity that defines where you start
Light entities so we can see the world
We have already created a hollow cube, so we have the sealed-off world part covered. Now, we just need a player_start entity and a light entity. Both of these entities are point entities and are created with the Entity Tool. Grab your Entity Tool from the Maptools bar (the knob icon between the Camera Tool
and Block Tool).
In the 3D view, left-click on any two random spots on the floor. This will create two copies of the default point entity at those points. With the Selection Tool, double-click on (or select it and press Alt+Enter) one of the entities you have just created to access the Object Properties. In the Class field, type info_player_start and hit the Apply button.
It might be pretty obvious, but the player will spawn at this location when the...