There's 2 steps for designing the report:
1. Choose your data source. To use an xml as a data source, just select it in the data source connection window.
2. Design your report using excel. Wherever you need data to be drawn out of the xml, insert the appropriate autotag. The tag reference can be found at the documentation page
in the autotag guide or the template creator's guide.
In step 2, if you just need a tabular report, then you can just drag the node that you want from the databin out into the spreadsheet and it will create the entire table automatically. However, if you need to do complex selections use the xpath wizard in the tag editor.