World Search is built into the basic UI and does not require an Addon. Use the BACKSPACE key on your keyboard or the globe icon on the minimap to open the World Search interface.
Click inside the black box to the left of the word "Search" and enter the name you are looking for. This can be an NPC, monster, or object name (but harvest resources cannot be searched for). Don't forget, you must type it in proper Upper and lower case, exactly as it appears in-game.