The "Sinumerik 840D" ude settings will influence the arc ouput with this post. To see that siemens ude - it may be necessary to "load" a machine. Once I loaded one of the siemens simulation machines and could select and set that ude - I could use the post you are speaking of out of postprocessor's directory and get circle output.
If you have already done this and are getting circular output but are wondering how to get XY's - force the XY words on the circular block (by highlighting and right clicking.)
The Sinumerik settings are probably set to linear output.
To see something similar to MILL_3_AXIS, try MILL_5_AXIS_ACTT_IN, and you will see the circle records.