Bentley has provided this wiki page for loading the SignCAD mdl in CONNECT products

https://bentleysystems.service-now.com/community?id=kb_article_view&sysparm_article=KB0018522

but the steps provided on that wiki page only works if your using the delivered workspaces, which changes are your using a custom workspace. 

 

A much easier way to configure your CONNECT product to load SignCAD for all configurations (delivered and custom) is described below.

1. Edit the OpenRoads SignCAD.cfg file found in C:\OpenRoads SignCAD\Ustn\CONNECT\ (this is the default install directory, if you chose a different install directory when installing SignCAD then use that Directory path to find this cfg file) to remove the # from the second to last line.

     OPENROADS_SIGNCAD_USTN_CONNECT = C:\OpenRoads SignCAD\Ustn\CONNECT/
     MS_MDL < $(OPENROADS_SIGNCAD_USTN_CONNECT)
     MS_SYMBRSRC > $(OPENROADS_SIGNCAD_USTN_CONNECT)*.rsc
     #MS_GUIDGNLIBLIST > $(OPENROADS_SIGNCAD_USTN_CONNECT)SignCAD Ribbon.dgnlib
     MS_DGNAPPS < SignCAD.ma

2. After editing the cfg file to remove the #, copy it to the products appl folder. for OpenRoads Designer the default directory is 

      C:\Program Files\Bentley\OpenRoads Designer CONNECT Edition\OpenRoadsDesigner\config\appl\

Relaunch/launch OpenRoads Designer and you should now see a workflow for SignCAD.