I'm not sure if this is what the OP was looking for, but this workaround solves my problems:
In Eclipse (Helios):
1. In the Package Explorer view, open the WindowTester Runtime library node.
2. Open the Properties dialog for commonRuntime.jar (select it, then Alt+Enter).
3. Open the Javadoc Location properties page.
4. Select Javadoc in archive and also External file.
5. For Archive path, browse to (precise name depends on Windows Tester version):
<eclipse install dir>\plugins\com.windowtester.eclipse.help_6.0.0.r36x201009141344\doc.zip.
6. For Path within archive, browse to html/reference/javadoc.
7. Press the Validate... button to make sure everything is OK.
8. Press OK.
Now the javadoc is integrated into Eclipse.
Update: You should repeat these steps for swingRuntime.jar, too.