Your folders should look something like this
Worldofwarcraft\interface\addons\quest helper
If you dont see that set up then your files are in the wrong place. When you extract your files if they are packaged correctly they should automatically be in a folder named after the addon your installing.
In this case Quest Helper.
Look in this folder for the files. You should see one called a .toc file.
If thats the case, just move the folder in to the addons folder so your file path is like above. When you log in you should see an addons button in the lower left hand corner of the character select screen. Click it, make sure your mod is there. Then log in and have fun =)
If your still having issues i would direct my searches to the following link:
http://www.wowinterface.com/forums/forumdisplay.php?f=3
The 3 stickies up top should answer any additional questions.