Hi Christian,
I was able to reproduce the problem. I'll get started on a fix for
http://bugs.mysql.com/bug.php?id=70408 but in the meantime here's the workaround that I used:
1. Uninstall the MySQL Installer through the Control Panel. This step is necessary.
2. Install mysql-installer-community-5.6.15.0.msi again (Sorry, I know this is no fun.)
3. Expect the Conversion dialog. Click No and allow the Installer to exit memory.
4. Edit the file C:\ProgramData\MySQL\MySQL Installer\config.xml. (See below for the important 5.6 gpl values.)
5. Save
http://wb.mysql.com/installer/products.xml to C:\ProgramData\MySQL\MySQL Installer\products.xml deleting the current one in the process.
6. Verify the file products.xml is at version 102 and the config.xml edits are persistent before moving to the next step.
7. Re-launch the MySQL Installer from the start menu.
The config.xml edits are:
<ActiveCatalog>mysql-5.6-winx64</ActiveCatalog>
<Commercial>false</Commercial>
If you don't want to make any hand edits, here's a confirmed working alternative:
http://forums.mysql.com/read.php?11,585821,586703#msg-586703