I have an Arduino Uno and ESP8266. I want to connect and control an LED from a web server. I made a connection referring to this:

I uploaded the WifiWebServer code to the Arduino, and applied my router's credentials.

char ssid[] = "Router SSID";      // your network SSID (name) 
char pass[] = "Router's WPA Password"; `

I'm getting a "Wifi Shield not present" message from the Serial Monitor. I even tried giving external 3.3v power supply to my ESP8266 module, but no change. What am I missing here? Can someone help me fix this issue?