I recently installed Unified Navigation in a 8.55 fluid environment. I installed the workcenter from the latest IH PUM Image (Doc ID 1641843.2). Here are some import/export DMS scripts and a App Designer project to help in migrating this to your environment. Big thanks to Simon Chiu for assistance with the Unified Nav functionality.
1) Run your export script (Click here to download) in IH PUM Image 3 to create the DAT file
2) Run the import script (Click here to download) in your IH environment to import the workcenter and activity guide
3) Copy the app designer project (Click here to download) into your IH environment
4) Bounce your app/web server for changes to take effect
5) From the Navigator menu go to Portal Administration->Unified Navigation Workcenter
-Note: You might have to add the Portal security role to your security if you get an error