One fairly common issue people have is that older addons they install do not load when they log into the game. This is usually due to the fact that World of Warcraft tags older addons as out of date and aren't automatically loaded unless you tell the game to. This typically happens after a patch, as the game client tends to mark addons as out of date at that point.
Check for Update
Sometimes following a patch addon author's will start updating their addons so that this isn't an issue. So check the addon's page to make sure that there isn't an updated version.
As you can see there is not an update to the addon in the example. As the newest file version is 5.1.6.
Loading Out of Data Addons
Just because an addon is out of date doesn't mean it doesn't work. Many times nothing changes in game that would result in the addon breaking, and in that case the author just needs to update the version the addon is under.
But this doesn't mean you can't still use the addon. You just need to let the game know you want to run out of date addons.
Log into the game and once on the Character Select screen click on the AddOns button in the lower left corner.
This will open up the AddOn List menu for you. Check the Load out of date AddOns at the top of the AddOn List window.
Though many times loading out of date addons won't cause any issues, if there has been changes to the game that affects the addon it could cause that addon, your game or other addons to not work properly. It is always better to install and updated version over loading out of date addons.