The admin pages can only be used to upload beans and plugins that consist of single files and work without an installer. The plugins you refer to from IT2Be and Dr. Maison & Partner have their own installers I believe, but perhaps they can comment themselves!
You can use our update manager to install using the developer.
We will soon make it possible again to use the update manager outside of Servoy.
Another option is to use the archive option in the update manager to create a zip file and upload that file (or structure).
The final option is to download our separate archive files (per plugin/bean) and follow the structure to upload to a server. However this is not the preferred/advised way to perform an update/install.