本文转自：http://www.developerbits.com/tag/unable-to-build-the-file-dx-jar-was-not-loaded-from-the-sdk-folder/
Problem While trying to run Android applications using eclipse IDE,
the following build error occurs: Unable to build: the file dx.jar was not loaded from the SDK folder!
Solution Quick fix is to
Right click on project -> Properties -> Java Build Path (From Left List)
-> Libraries (Tab) -> Add Library (Button on right)
-> JRE System Library -> Next -> Workspace default JRE
-> Finish Permanent fix is to install latest Java SDK.