Move the line into setup(), don't leave it at the top of the sketch (and start using code tags, the </> button):
int buttons[6]; // creates a global array with 6 elements, no values assigned
int notes[] = {262,294,330,349}; // global array with 4 elements, values assigned
void setup() {
Serial.begin(9600);
buttons[0] = 2; // puts a value into one of the elements
}
[/code]