tvogel,
1)Can you share your build id from Help > About MyEclipse Enterprise Workbench > Build id? Paste it here.
2) Matisse4MyEclipse only supports Swing but you could install the WindowBuilder plugin using the MyEclipse Configuration Center. The related software for that has a SWT designer.
Here is a thread which lists the update site for WindowsBuilder according to the base version of Eclipse for your MyEclipse (Ex : If you are working on ME 9.0 then use the update site for Eclipse 3.6).
http://code.google.com/javadevtools/download-wbpro.html
3) Here are the steps to install it using the update site :
1. Open MyEclipse IDE, from menu options click on Help > MyEclipse Configuration Center.
2. Open Software tab
3. Click on “add site”. This will open Add Update site window. Add the update site url and name.
5. Expand WindowsBuilder under personal sites till the child nodes. Right click on each child node and click on [add to profile]
6. Click on [Apply x changes]
7. Accept software licenses when prompted
7. This will open a wizard which will guide you through the rest of the steps
8. When prompted, please choose to restart the IDE
4) Here is a thread which explains install through update site with screenshots :
https://www.genuitec.com/forums/topic/android-adt-installation/
Let me know how this works for you.