many
There are a few possible reasons why the addon does not work as it should: 1) The addon has to be properly installed (in the WoW\Interface\Addons folder) and has to be checked in the list of addons under the "Addons" button at the character selection screen. 2) The addon might be out of date, since patches regularly break the functionality of existing addons. Download the latest version of the addon and install it. Also check if there's a red error message "Dependency Missing", which indicates it might need another addon installed and running to work. 3) Check if the configuration is done properly. Most addons have a console built-in, but you should also be able to access it via Escape -> Interface -> Addons. Make sure that it works properly and is set up as you want it. 4) Other addons might block the functionality. For instance, having another addon that replaces a frame this addon requires (such as button bars). Check on the webpage where you downloaded the addon if such is the case, there's usually also ways to fix it. If none of the above fixes the issue, try to contact the author of the addon through the webpage where you downloaded it. Usually there's an attached forum or comment boxes - here you can also tell the author of problems you are having, and may get an answer.
download the addon. Should be on a zip file. Simply extract the file. place the folder with all the files in your >world of warcraft>interface>addons folder.
You need to have addon
You need to put the addon in to your addon folder. The easiest way to do this is download the curse client (free) and then anytime you want a new addon just install it via the "Install VIA Curse Client" button.
GET FIREFOX, (IF YOU DONT HAVE IT) DOWNLOAD THE ADDON ILLIMITUX AT https://addons.mozilla.org/en-US/firefox/addon/11037
the only jeapordy addon found is linked below to the Curse addon site. It adds some sounds for certain events.
You need to put the addon in to your addon folder. The easiest way to do this is download the curse client (free) and then anytime you want a new addon just install it via the "Install VIA Curse Client" button
Yes, you can, however, not very many people would know about them. Most people tend to stick to the major addon sites that have tools that help addon authors. Sites like Curse and WoWInterface are better equipped to serve your addon to large audiences.
Download the playermodel addon from the workshop
A must have addon for all raiders in World of Warcraft is Deadly Boss Mods, also known as DBM. This addon will cause warnings to pop up on your screen when important things are going to happen. An example of this would be on the first boss in Heroic SFK. DBM will tell you to interrupt his self heals and will also warn you about when he about to transform towards the end of the fight.
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.
I would start by browsing the UI & Macros forum at the site in the related links. It is on the official wow forums and will give you a huge head-start in your addon development.