In FDT, you can find the list linked libraries in the .as3_classpath file if those libraries are in your project folder (or below). They reflect the content you add in the 'Library' tab of 'FDT Build Path' in your project's properties. There's also an entry in the .project file that reflect the linked libraries you add with the 'Source' tab. Those libraries can be located anywhere on your hard drive, and that's why you define 'path variables' for them.
Personally, I create a 'lib' folder in my project where I copy the libraries I need and add them with the 'Library' tab. I find it much easier to share the project (since everything is relative to the project) and you don't need to worry about having two projects modifying the same library file.
For the location of those 'Path' variables, as far as I know, could only be located in your eclipse installation folder or in your eclipse 'settings' folder, usually in your 'user' directory.