Once you have installed the Datasmith Exporter for Archicad, a Datasmith toolbar option is added to the Windows > Palettes menu.
Enable the toolbar using the Windows > Palette menu option.
Once enabled, clicking the Export to Datasmith option will open the Export to Datasmith file window.
Export content from Archicad using the Export to Datasmith button.
Browse for the location in which you want to save your exported file, set the File name, and click Save.
End Result
Your .udatasmith file should now be ready to import into Unreal Engine. See Importing Datasmith Content into Unreal Engine 4 for more information. If your data needs additional cleaning, merging, or other modifications during the import process, see Visual Dataprep.