Once you have imported the model as an asset, you can place it in your scenes like you would any other Static Mesh. The most common methods are:
Place them manually. Simply drag the asset and drop it in the scene to place a model. This gives you the most flexibility in how it is lit. Setting the mobility to Static will disable the ability to move the SpeedTree at runtime but will allow it to be lit by Lightmass in addition to dynamic lights. Setting it to Movable will enable you to move the SpeedTree around at runtime but it will not be able to receive static lighting.
Paint them as foliage - Drag the asset to the foliage painting area and paint the models like any other Static Mesh.
Painted geometry is instanced and renders very quickly; however, you will need to enable Lightmaps for the instances and possibly set a lower lightmap resolution. Keep this in mind as you populate your scenes.