Yes, you will need to add ":toolbox-button t" (without the quotation marks) to the top of your dialog if you want the button to appear in the toolbox.
So the top of your dialog definition could look like:
Code:
(sd-defdialog 'iwg-teil-mehrfach-eigenstaendig-dialog
:dialog-title "Mehrfach Eigenstaendig"
:toolbox-button t
;;:dialog-control :sequential
...
Does that help?