You need a '.arena' file for it to show up in the list (Just look at the ATCS one). Also make sure the map is in a 'maps' folder.
Just in case you didn't know, you can also run a map via console commands. ('[wiki]map[/wiki]' and '[wiki]devmap[/wiki]')
Better yet, you can create a shortcut with launch options so your map starts up immediately.
EDIT: Here's an old shot I made (You don't need everything shown but this just makes it clearer);
