Move the mouse to where you want to place a tile. Press the (left) mouse button, and move the mouse over the rack while keeping the button down. Then double-click the tile you want to place on that square. To remove a tile, just mouse to it and click on it once.
Known bugs: 1) it is possible to remove the premium squares from the board.
2) This only works under Netscape. There will be a different mechanism for Internet Explorer users.
3) The first time you use it, it takes a little time to load all the icons. After that it's not too bad.