Turn on the Finder extension and locations
M-Client switches on its Finder extension at every launch. If someone turned it off, M-Client Settings shows an orange Turn On… bar.
The right-click menu
Open System Settings ▸ General ▸ Login Items & Extensions and turn on M-Client Finder. On recent macOS versions it is listed under Extensions ▸ By Category ▸ File Providers.
The Finder locations
In the same place, open the File Providers entry of M-Client and switch on the connections you want in the Finder sidebar.
The exact wording of System Settings differs between macOS versions. Search System Settings for "Extensions" if you cannot find the entry.