So youve placed your regular prop_physics or whatever in your map, but when you compile it, you dont see it. That is due to a resource-conserving feature called the "visibility sphere". When outside of the visibility sphere, you cannot see the prop. When you are in it - you guessed it - you can see it.
1. Right click your prop and click "properties". Notice the extremely small circle right under the chair's "axles" or whatever you want to call them - thats the visibility sphere.
2. Scroll down in the left side of the window to "Start Fade Dist". This defines how far away you need to be for the prop to be invisible. The higher (or lower if negative) the number is, the bigger the visibility sphere. It defaults to -1. We want no visibility sphere so that you can see the prop from anywhere on the map, so set this to zero.
3. The next is optional, the fade scale. This is basically a flag to determine whether or not the model fades out. DirectX7 cards automatically cause props to fade unless this flag is set to 0. Numbers smaller than 1 cause the prop to fade out at further distances, and greater than 1 cause it to fade out at closer distances. Setting it to zero turns off forcible fades, but adds to the budget.
And viola, it shows up in game. I hope this makes sense. If you get to be an advanced mapper, you will want to learn about when to use the visibilty sphere instead of just getting rid of it. But for now, youre OK.