Hi Wolfgang,
locally you can open a specific version by using the Mendix Version Selector, which is by default installed when installing a modeler.
Located at "C:\Program Files\Mendix\Version Selector", note it will only show installed Mendix Pro versions.
If you don't have 9.12.4 installed, you can download it from the "Get studio pro" page.
If this doesn't solve the problem, which appears to be, not able to add buttons. Please let us know and add some more details (screenshots and explanation) of the page