The Bancontact functionality will be available from Magento version 1.22.0 of the PayPlug module.
For activation, simply go to your Magento back office and activate the feature as shown below : Stores > Configuration > Sales > Payment Methods > Bancontact > Enabled > Yes
Add the title : Pay with Bancontact and then save changes
To find out more about Bancontact's functionality you can read our dedicated FAQ