Creating a new Dimension for Datasources can be done by pressing the "Settings" button on the top right corner as it will open the Settings of your current Datasource. You will see a button "Add Dimension" which allows you to add a Name and a Key (German / de for example) to define a new dimension. After adding your required dimensions press the "Save" button on the bottom of the form and you will see Dimensions being added above the list as small bubbles.

By clicking on one of the Dimensionbubbles you will see a new input field being added to the list besides your default input field for the values per key.