Place a prop entiy. Moveable props are prop_physics, stationary props are prop_static. If your prop's not showing up, it's probably the opposite kind. Animated props are prop_dynamic, but we won't go into that.
Place your prop entity where you want a prop.
Open the properties. Under the model field, click the browse button.
Open the desired props folder. It will mount the GCF, no need to extract anything.
Pick a prop. Click Ok.
Click Apply.
Done.
The thumbnails offer a little view of the prop, I've got a listing of CS:S props with images here:
http://rabidmonkey777.bluewolf72.com/imagery/prop_images