There is an easy way to connect multiple MPI partners together, that is using Profibus-DP cable and connectors. Simply daisy-chain the MPI ports of the PLC and HMI units with these. Make sure that all units have a distinct MPI address, of course.
When you want to connect you PC cable, you should make sure the Profibus connector that you install on the PLC has a "piggy-back" connector, that is a sencondary female DB9 on the back of the connector where you can plug the male DB9 of the PC-Adapter USB cable; the PLC port provides power for the Adapter circuits (although there is an optional power supply you can buy for this, it is easier if you plug directly on the PLC port).
The Siemens Profibus connectors that come without a piggyback connector have a part number like 6ES7972-0BA50-0XA0; the same model with piggyback connector is 6ES7972-0BB50-0XA0.
Hope this helps,