Left 4 Dead video tutorial shows you how to create a survival gametype.
Setting up all the necessary entities and in part 2, I cover nav_meshes for survival map.
1. Name your map as l4d_sv_yournamemap. Save it.
2. Insert the following entities.
3. Duplicate info_survivor_position 3 more times. You should have 4 total.
4. Double click on info survivor position and name your survivors accordingly. Such as Zoey, Bill, Francis and Louis.
5. Insert a prop_static to act as a button. In our case it will be a radio.
Under world model in prop_static window find hamradio.mdl by typing into the filter option.
6. Create a brush with nodraw texture over the radio.
Select the brush and press CNTRL+T to tie it into an entity.
Select func_button as the entity class.
7. Double click on the new func_button and go to output tabs.
My Output Named: onPressed
Target Entity Named: director
Via this Output: Panic Event
Fire Once Only: Checked
7. Create two more entities.
My Output Named: onMapSpawn
Target Entity Named: survival
Via this Output: command
With a parameter override: mp_gamemode survival
8. Name your map as l4d_sv_yournamemap. Save it.