hi yea tuning can be difficult but once you get it becomes second nature.
iam not sure if u went to the site but go to the cen's faq site
http://www.cenracing.com/faq.html and ull see at the start of the page 'engine tuning guides' heading , just click the nx 15 link,
the black screw on the bottem where you caant reach witht he screw is the idle screw, normally u dont really have to muck around with that. ok the screw on the carb its the biggest one, coloured brass a bit and is where one of the fuel lines goes to is the high speed, the low end screw is located in front of the carb where it opens (thers like a black tube thing where one of your servo linkages is connected)
also you might wanna follow that manual its got some important tips.