No, most addons will not get you banned. The only things that addons are allowed to do in game are approved by Blizzard. If Blizzard does not want an addon to do something, they just make it so that addon can not do it.
Healbot is an addon, which means it is third-party software. It is most often downloaded from a game or addon site like Curse, WoWInterface or the like. If you are unsure, check if any of your online friends use addons, and where they get them. You can also google "healbot addon" and find the download results.
Download a wow addon manager like wowmatrix. Google Wowmatrix --- Inside your World of Warcraft game folder you will find a folder called "Interface". Inside is a folder called "Addons". Addons provided on abovementioned websites usually come zipped, with the files in a folder with the addon's name. Simply copy this folder entirely into the "Addons" folder. Make sure to quit WoW and restart it. At your character selection screen, at the bottom right, is a button "Addons". Click it to see a list of addons you are using. Make sure all addons you want to use have a tick in the box, and that the box "Load out of Date Addons" is ticked, in case you installed an older addon. Log into your character, and the addon should now be active.
You have to find an addon first that is supported by cataclysm. Then once you have one you put it in the "Interface" folder in the world of warcraft folder. There will be a folder in Interface called "addons" thats where you put the addons. then before selecting your character when you are almost in game you need to click on manage addons in the bottom left corner and make sure they will be on. one good place to check out for addons is the curse addon site listed below.
Any addon is legal as long as it follows blizzards guidelines. Hence, most addons are legal.
The Quest Helper mod for World of Warcraft can be downloaded from the Curse website listed below. You do not need the curse client to install it to your game, although it makes it easier. Just install it like any other addon, to your addons directory of your WoW interface folder.
No the PS3 game addons must be downloaded from the Playstation Store
All addons for World of Warcraft can be found in the Interface\Addons folder inside the game's folder. If you put the game on an external harddrive, addons should be put in that same folder on the external drive. If you simply want to copy the addons over, simply copy the Interface\Addons and the WTF folders - they contain the addon modules and the settings, respectively. Make sure to change the name of the Account, Realm and Characters inside the folders if you are planning to use them for another account or realm.
when you download an addon, put in in program files, steam, steamapps, <your user name>, garrysmod, garrysmod, addons. extract all files to this folder.
The easiest and best way to use coordinates is to get an addon for the game. Then when someone gives you coordinates to go to, you can just tell the addon to point the way. One of the more popular addons for coordinates is called TomTom (linked below). It includes an arrow that literally points you in the direction of the coordinates.
Some people have made such guides, but they sell them on the internet. If you need help on a specific quest, there are sites like WoWHead which will give you the answers you need. And finally, if you want to get as many quests done, you can install the addon called Questhelper to get a convenient help function inside the game for it.
First off, make sure the addon is in the correct location. The whole "Gatherer" folder should be copied into World of Warcraft\Interface\Addons. Otherwise, it will not work. Since you say that the addon does not show up when you press the button in the bottom left corner, I assume this is the issue. Inside the game you should be able to configure gatherer either by typing /gatherer, or by hitting Escape, and going to the Interface menu. Try and find the options for Gatherer there.