Load cells are typically Wheatstone bridges of 350 Ohm strain gage resistors. So you will need to power the bridge itself with between between 2 and 10 volts. It will use a fair amount of power, more than a battery will give for long, because of the bridge resistance being only 700 Ohms. The output will be in the millivolt range. So you'll need to amplify it by up to 500X, probably with a differential input op-amp. I'm no Arduino expert, being just gotten into it as an interest. But I've used load cells in my profession for 20-some years. Tell me the make and model of your load cell and I could possibly calculate your requirements.
In the photos of load cells on your link I see four wires: Red, Black, Green & White. Refer to this Wiki on Wheatstone bridges...http://en.wikipedia.org/wiki/Wheatstone_bridgeRed = Power, Pos (A on the Wiki)Black = Power, Neg (C on the Wiki)White & Green = Differential Volts Output (B & D on the Wiki)Supply your power to Red & Black. Connect White & Green to the + & - inputs of ANY differential input operational amplifier IC (or multi-chip IC circuit). You will get amplified volts out. Get any OpAmp cookbook for examples in differential amp circuits. They are simple enough: one chip, four resistors.I am not the one to ask (yet) about Arduino code. I know only LabVIEW, Perl & PostScript having forgotten all my Forth and Fortran. Others here will have to answer your code requirements.
You will get a voltage that varies with the weight on the load cell. Here's a complete example: http://cerulean.dk/words/?page_id=42
You would need to test your load cell with a known 2kg weight to see what measurement you get and then use that measurement in your code.The full calibration process would typically entail testing the load cell with a number of different known weights and then building a graph of weight vs measurement -- and then building a function to calculate the weight for any given measurement. I would recommend using a spreadsheet (Excel, etc.) to record those values and build the graph because then you can also use an Excel feature to create a "line of best fit" which will give you your function.
But i really need to get a weight measurement. That is not in a graph.