You need to buy a code. For one payment of $19.95
Once you've payed and the Money has been recieved, you will recieve your Activation Code a little after.
You enter your code on the game, and are granted the ability to play online, and can build more than the default [150?] bricks.
Otherwise, you just play the Demo, which grants you single player mode and the ability to plant [150?] bricks.
But you can still download Add-Ons and such from the Forums/RTB/Other places.