Working With Linked Resources
From FDT Documentation
Some developers like to have source code, either via a folder or .AS files, added to their project that is saved on their workstation outside of their project. The solution for this is to use FDT's 'Linked Resource' feature.
Adding & Creating A Bookmark
Begin by right clicking on your project and selecting 'Properties'.
FDT will then open up your project properties panel.
Once here, if you have already created bookmarks for resources, you can choose them. If not you can click 'Add' to create more resource bookmarks.
After adding the resource to the build path, you can chose to have FDT show errors in the Problems and Explorer views.
By default, Linked Resources are locked and can't be edited. If a developer wishes to directly edit a file, she will need to enable edits in Linked Resources.
This is done be going to Preferences>FDT>Misc>Enable working in linked libraries