To use the Newgrounds API with the command-line mxmlc compiler, you must add the NewgroundsAPI.swc to the library path.
Copy NewgroundsAPI.swc to your project folder.
Unzip NewgroundsAPI.zip, and copy the SWC file into your source folder.
Add the NewgroundsAPI.swc library to the mxmlc command line.
Use the library-path command line option to link your project to the Newgrounds API library. For example:
mxmlc MyClass.as -library-path+=NewgroundsAPI.swc
You are ready to go!
The Newgrounds API is now ready to use.