Create an entity called "prop_static" and click on the entity and press alt+enter to bring up the properties screen. Then, click on "browse" to choose the particular prop you want to use (tree, truck, whatever) and then once you've chosen that you can click "apply" and the model should show up in your 3D view. Also, if you want to do further tweaking, check out the "flags" tab and make any adjustments there that you'd like.

Bear in mind, models that are big and don't move are usually prop_statics, and models that are smaller (like TVs) that can get picked up or thrown around are usually prop_physics. Or if you're creating a multiplayer map they are prop_physics_multiplayer (these are less performance intensive and are necessary for online play). If you add a model and it doesn't show up in game, most likely it's the wrong prop type.

Hope that answers your question, and if it does, please mark my answer correct so others can use this thread as a reference.