How can I Control+Click on FlexUnit types to see what is going on behind the scenes?
Adobe does not include the code for FlexUnit 4.x in Flash Builder 4 Premium. Only the library is included.
If you want to take a look at FlexUnit internals, you can download our code at flexunit.org, go into the build path/library path of the project, find the place where they link in the flexunit-core libraries and edit the source attachment. That will allow the Control-Click to work, etc. The 4.0 version on FlexUnit.org is the version that shipped with Flash Builder 4 Premium.
We recommend updating to FlexUnit4-RC1, which is also available for download at the Github.
Furthermore, check out the download page for the Turnkey Project. The Turnkey Project is a simple self-contained project to help users and potential developers learn more about FlexUnit technology.