First, head over to https://app.datatrics.com/channel and search for MailPlus.
Click on the green "Integrate" button and you will be redirected to the following screen:
Login to MailPlus and go to the tab "MailPlus settings". Afterwards click on "MailPlus REST API" and copy the Key & Secret into the fields Key & Secret in Datatrics.
If you want to update your MailPlus profiles with Datatrics fields select yes.
After saving the settings Datatrics will synchronize all Contacts.