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Place the 'BabylonJS' folder containing the .unitypackage in your Unity Asset Store folder:
Mac: ~/Library/Unity/Asset Store-5x/
Windows: %AppData%\Roaming\Unity\Asset Store-5.x
Example: ---> Asset Store-5.x
---> BabylonJS
---> BabylonJS Editor Toolkit.unitypackage
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When creating a new unity project, select 'Add Asset Package' and enable 'BabylonJS Editor Toolkit' support.
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When using an existing unity project, select 'Assets->Import Package...' and choose your 'BabylonJS Editor Toolkit' unity package.