The best way to show advertisements in the server would likely be with a Sourcemod plugin. I don't know one off the top of my head (apart from the built-in message displaying system), but I'm sure there are a ton
Mani can use one of a few lists for determining which maps can be voted for. You might try adding your map to mapcycle.txt or maplist.txt, as these are most commonly used. You can define the file choice itself in your mani.cfg file -- "mani_vote_mapcycle_mode_for_random_map_vote" is the cvar to look for there.