Welcome, fastline!
I don't play on a mac so I'm not sure how that works, but I'll try to help you to the best of my knowledge.
The basic idea of the game is this:
You are the white @. Other players will be other @'s, different colors based on their classes. Most other letters will be monsters, some symbols will be terrain features, such as . being floor and * being trees. Basically - if it moves, it's a monster, kill it! The final monster is Morgoth, Lord of Darkness. He's a black P and you won't see him for a long time, it could take many months or years. But believe me when I say that there's enough other monsters to keep you busy until then, it just never stops :)
So yes, you begin a new game, start up the client (mangclient.exe) and connect to mangband.org. Enter the character name and password that you want to use and select race/class/stats.
You then start in the tavern of the town, building 8. You move with the arrow keys. If you have a
numpad this is the best option. It's also possible to use
roguelike keys. The keys to move are 'h'/'j'/'k'/'l'/'y'/'u'/'b'/'n' I believe.
I mentioned that "if it moves, kill it!". The most obvious way to kill a monster is to move next to it. Once a monster is within the reach of your sword, you'll automatically attack it. This is due to the realtime character of the game. There's also a few monsters that don't move, but they'll typically be a letter, like 'm' or 'j'. You can look around with "l" to check out whats around you. Also "L" to get a bigger view.
The blocks in town with numbers attached to them are stores. You can move onto them to enter the store. You can buy supplies for your adventure, helmets, scrolls, magic books and so on.
Some find the learning curve of the game a little steep, especially if you've never played a so called roguelike game before. But, since you seem determined to figure it out, I'm sure you'll do ok.
One final thing, it's possible to chat in the game by entering the ":" key. You can just ask players for help in the game also, as you run into stuff you don't understand. Most players will be happy to help, we're quite a friendly community.
Cheers,
Warrior