There are few online Lego Creators you might want to try. They will all be similar to Lego Digital Designer, but also have their own unique features and controls. Look for an helpful "tip" or instructions before you start tinkering since knowing the quick keys will make things much easier :)
Lego With Chrome: Publish Your Design
http://www.publishyourdesign.com/design
Meca Bricks
Lego Digital Designer
This one is not online, but it is free and has a version for Mac and PC.
This is the program we use in class.