Here's a VERY simple routine to see what you've got everything working:

#include "FastLED.h"
#define NUM_LEDS 20

CRGB leds[NUM_LEDS];

void setup() {
  FastLED.addLeds<WS2812, 12>(leds, NUM_LEDS);
}

void loop() {
  leds[0] = CRGB::White; FastLED.show(); delay(10);
  leds[0] = CRGB::Black; FastLED.show(); delay(10);
}

If this doesn't compile and run, please see our General Troubleshooting page.