When I download a Halo 2 (PC) map from this site it never lets me use it. I copy and paste like with Halo Custom Edition but nothing appears.

While I'm here can anyone tell me how to play Halo Custom Edition Campaign maps.

Did you put the Halo 2 maps in:

C:\Users\USERNAME\Documents\My Games\Halo 2\Maps

to answer your question about halo custom edition campaign maps...

first of all make sure you have -console added in your halo custom edition game properties.
to do that right click on your halo custom edition icon on your desktop and click on properties.
then you will see text that says...
"C:\program files\microsoft games\halo custom edition\haloce.exe"

left click at the end of that text and add a space then -console
so it should look like...
"C:\program files\microsoft games\halo custom edition\haloce.exe" -console

when that's done click apply and click ok

now when you download a custom edition campaign map you will find a read me document inside the folder,this will tell you what the map is called and what to type to start the map.

so if the name of the campain map was for example...
b_40
then you would press the ~ that is the tilde key it's the key beside 1 and above tab
then you would type map_name b_40
and press enter,the map will load and start,to quit the map press the ~and type quit then press enter.