The older web UI had its own tab for MPPS. The Web3 UI has MPPS integrated into the study lists under the 'Folder' tab e.g.
Select the 'Link' icon to link with a MWL item.
Select the 'Modality' icon to emulate MPPS for the study.
Note also that there is a drop down menu in the search panel that allows you to list unlinked studies i.e.