Thank you for your interest in Aspose.Words for SharePoint.
Please make sure that the following line:
is present in the web.config and the assembly mentioned is installed in the GAC. This actually should be the result of the setup.
If so, you can try uninstalling Aspose.Words for SharePoint to verify that the problem is caused by the installation. Remove Aspose.Words for SharePoint and verify if the problem still persists when creating other workflows in SPD.
Also it would be helpful to know which version of operating system, SharePoint and SharePoint Designer are you using. I assume you are using the latest Aspose.Words for SharePoint (1.4.0).