If you have Curse Addon Client - you can use it, as it's supporting Wildstar.
To install addons manually type %appdata%/NCSoft in adress bar of "My Computer" (or any other) folder, like on screen, and then press Enter.
There find Wildstar folder. Then go to AddOns folder, if there none - create one.
Unzip folders you downloaded on Curse inside AddOns folder. It must look like on screen:
If AddOns are not loading or outdated, make sure you set them to load manually. Pres ESC, there go to AddOns and select ones that you've installed. Don't forget to disable default AddOn called "CombatLog" - this will save your FPS inside raid groups.
After that press the button "Reload UI" or type command /reloadui in chat.
Essential AddOns for raiding:
RaidCore — will show you timers, draw lines to you and announce meaningful mechanics.
Interrupter — will show you interruptable casts of bosses, so you can easily interrupt them.
Vince Raid Frames — replacing default raid frames with customizable frames.
Good addons as well are: Misch Essences, Perspective, TapThat, Combat/Dark/Vortex Meter.
Forge UI or Kurona Frames + Optiplates are good choice to replace dafault UI (will raise FPS as well).