What is the difference between this mod and Creative Mode?

Creative Mode is a very old mod, and therefore does a lot of things inefficiently. For example, Creative Mode's active energy source is actually a disguised steam engine. Every single tick, the mod manually fills the engine with output energy. This is extremely slow and does not scale well at all.

By contrast, EE's infinity accumulator can achieve the same effect using an electric energy interface, which is an entity built in to the game that magically never runs out of energy. It removes all of that extra processing, and is therefore over an order of magnitude faster.

Additionally, EE leverages the power of the game's built-in map editor. Using the map editor, you can do almost everything that Creative Mode allows you to do, all with much better performance and flexibility.

How do I access the testing items?

The testing tools are available while inside of the map editor. You can toggle the map editor by using the shortcut bar button, or with Control + E.

But I don't want to use the editor!

You can also access the testing items outside of the editor by using /cheatmode or /cheatmode all. However, things like instant blueprint building and changing game-related cheats can only be done in the editor.