Before using the Model Driven Development Environment:

You should be using the correct edition: Enterprise Architect Professional, Corporate or extended editions
You should be connected to the required model
Relevant source code files should be linked to Classes in the model (sufficient to be editable from the internal code editors) either:
Using MDA Transforms to generate stub code files, and internal editing to write code
Reverse engineering source code from existing files, or
Importing existing source code from external files
Required External Frameworks or compilers should be installed and operable
An Analyzer Script should be configured to link to the compilers in the External Framework

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Learning Center topics

(Alt+F1) | Software Engineering | Import Code
(Alt+F1) | Software Engineering | Code Editor | Using Intellisense
(Alt+F1) | Software Engineering | Code Editor | Browse to Source File