The connector shows the attributes for which editable input widgets can be made. If you drag in an attribute from OrderLine, you will get a reference selector which allows you to change the reference of the comment to another order line. This might not be what you want if you just want to display some data.
To get a read-only display and reach attributes further down the line, you can add a text box by hand - from the toolbar or toolbox - and then look at the connector again. Now, you can drag in attributes of entities that are two associations away from the current object. Or you can right-click the widget and choose "Select attribute...".
The result will look something like this, where the second widget is the reference selector you would get if you simply drag an attribute of OrderLine.