ScriptingRT is open source, and there might come a day when you want to add or change it. If you do that, you have to compile the source code again into the library. Here is how you do that using FlashBuilder.
In FlashBuilder, create a new Flex Library Project.
Move the org folder from the downloaded source code and everything in it to the source folder of your new project. Usually, the source folder will be called src, so you will have a folder structure MyScriptingProject/src/org/nad/… . Make sure that you do not create something like src/src/org when you copy the source code folders.
ScriptingRT uses two additional libraries that you need to include when compiling.
1. The D.Eval Library
Download the zip file containing the library, extract the swc file (currently D-eval-1.1.swc), and put it in a convenient folder (not the project folder itself)
Link to that file in Flashbuilder via Project/Properties/Flex Library Build Path, where you click Add SWC and point it to the .swc file of that you just unpacked.
When you are finished with this, the .swc file should be listed as a Referenced Library in your Package Explorer view.
2. The AS3Eval library
Download their source code zip, unpack.
You need three files from the com/hurlant/eval/ast folder: IAstNamespace.as, IAstReservedNamespace.as, and ProtectedNamespace.as. Importantly, you need to keep that folder structure. In your src folder, create subfolders hurlant/eval/ast/, and put the three files there. They will show up in your Package Explorer.
Your project should look similar to this:
If everything worked, you should have no errors in the problems console, and be able to build the scriptingrtxxx.swc library
More on linking ressources in FlashBuilder