Inserting a field is the common way to get data from the XML source into the document. To insert a field you can either just drag and drop the data element you would like to show from the data source window or position the cursor where you want to place the field and select Insert->Field. Again, you will be asked for an XPath expression. It is important to note the Context that is printed at the top of the window, as the XPath expression you create will be relative to this.
In this example you are in context of a room, e.g. you have created a section or repeating table for each room. You can then navigate to the attribute you want by e.g. expanding core and double click on the room_func_no attribute to get the room number. This will construct the following expression: