Nzp custom maps won't load

I'm installing the maps by putting them in the map folder but when I enter the game it doesn't show it and say that it is not down and doesn't exist

TheANTWON wrote:I'm installing the maps by putting them in the map folder but when I enter the game it doesn't show it and say that it is not down and doesn't exist

You need to use the console. From the main menu, press Esc (Escape) or ~ (Tilde) to make the console appear. (~ makes it come half-way down and is used in-game, while using Esc is much quicker when you just open the game to play a custom game)
From there, you type:
map mapname.bsp
I'm pretty sure you can also type maps for a list of maps, to make it easier. This makes the game start in the selected map. (If you are debugging a map you would type:
sv_cheats 1
map mapname.bsp
This allows you to fly and noclip around. It helps to check stuff quickly)
Hope this extremely detailed explanation helps.