Hook everything up EXACTLY like in this picture from the Arduino page:

http://arduino.cc/en/uploads/Tutorial/lcd_schem.png 
Now load the "Scroll" Example from Arduinos LiquidCrystal Library.
Change these two lines

lcd.begin(16, 2);
lcd.setCursor(0,7);

to

lcd.begin(8, 2);
lcd.setCursor(0,0);

now upload it to your Arduino or Seeduino and watch your display fire up!
You may have to adjust the potentiometer a little if you can't see anything.

As an alternative to the potentiometer you can connect the Vo pin of the LCD with any available PWM-output (I use pin 9) and add the following code in the setup() section:

pinMode(9, OUTPUT);
analogWrite(9, CONTRAST);

replace CONTRAST with the value of your choice. Values around 10-20 give a nice bright output.