By clicking "Build" you open the 3DHive Builder. The builder is the place where you can access the game world and modify it any way you wish using the available tools and assets. Using its simple point-and-click interface allows you to focus on the content and design of games while leaving the programming to the software. It also has an inbuilt logic editor which saves you time by providing you ready-made art assets, customizable avatars and maps.
Clicking builder will prompt you with this screen.
Select Create New Map to open up the level editor.
Give your level a name and choose a theme. Themes will be covered in greater detail later on but for now just know that the theme dictates the overall look of your level and all the natural features found in it such as mountains and rivers.