Google or you might be able to find it on twhl, but in a fellacio, you have to target a func_camera (or whatever its called) with your button, then you have to target a info_target your camera...
Killer-Duck Homicidal Anatidae
2008 Mar 5 • 1169
633 ₧
Create an info_target(point-entity) and give it a name. Create a trigger_camera(point-entity) and make it target the info_target, also give the trigger_camera a name. Create a func_button(brush-entity) and make it target the trigger_camera.