Here is the solution:
- Highlight one of the imports
- Control-click / press f3 to go to the declaration
- Click 'Link with Editor' on the Package Explorer view
- Right-click->Properties / press Alt+Enter on the parent jar (org.apache.tuscany.sca.api.jar)
- Select 'Javadoc Location'
- Set the Javadoc URL to http://tuscany.apache.org/doc/javadoc/java-sca-1.6.1/sca-api/
- Validate the location to be sure you didn't accidentally include index.html at the end (which I always screw up)