I've created an addon Folder and added a custom map. but it didn't Show
up in the map section.... so what do I have to do?

Might be you set it up wrong, but you have another option.
Make sure your addon folder is just inside "Star Wars Battlefront II PC Server" and the 3 letter mods are in there.

Some time's I've known my server not to show addon levels, but they are there.
Go join the server and use an admin command to run the map you want to play.

/admin /login youraddminpassword
/admin /changemap ABCg_con

It should play, it the server crashes "could not open mission.lvl" then it's out of my knowledge.