Right this is expected (AFAIK). Search the forum for the QtWorkBench plugin. 
... the plugin isn't on the web page, the attachements in the forum are deleted and the thread only mentions frequently that a new version is uploaded but no URL as far as I can see.
in the meanwhile you can download the plugin sources from here
http://tiwag.cb.googlepages.com/qtworkbench-0.4.2.alpha.zipyou have to build it yourself, create a folder in the CB source tree,
for example, i have it set up in : CB/trunk/src/plugins/_other/
and copy all files and subdirectories there, so that you have the following tree
CB/trunk/src/plugins/_other/QtWorkbench
CB/trunk/src/plugins/_other/QtWorkbench/res
CB/trunk/src/plugins/_other/QtWorkbench/src
then open CB/trunk/src/plugins/_other/QtWorkbench/qtworkbench.cbp
with CodeBlocks and build it. it will be built in CB/trunk/src/devel/share/CodeBlocks/plugins
after you run the CB/trunk/src/update.bat (on Windows) it will be copied to
the CB working version tree CB/trunk/src/output
i tested it with Qt 4.1.2 and generally it works fine.