The map editor and the model viewer share one client, so you can move between them without opening anything twice.
From a spawn to its monster
Select one spawn on the map, then choose Tools > Open the selected spawn's model. The monster it places opens as the model tab, ready to animate, capture or recolour. The map stays open in its own tab.
From a model to the map
With a map and a model both open, Tools > Place the open model on the map brings the map to the front and starts placing that model as a prop: click the map where it should go. It works where the model is one of the game's own prop models, the ones the Place panel lists; a character or a creature can't be placed as a prop.
A model standing in a map
On the model's Scene tab, set Background to Game map and pick a map. The model stands in the map's town or at its start point, on open level ground, lit by the map. See Camera, background and lighting.
Textures from a model
Tools > Recolor the model's textures… opens the open model's textures for recolouring. A saved recolour shows on the model at once, and on every map that places it once installed. See Recolouring textures.