Exporting the data is easy. From the main Dexcom Software menu, select Tools --> Export Sensor / Meter Data:
You can export all readings, or choose a subset. You can select sensor or meter readings (you have to export them separately and merge if you want them in the same file). You can choose an XML file, or tab-delimited TXT file. Defaults are all readings from the sensor in an XML file. Here is the dialog box for exporting data:
Here is an excerpt of the XML file containing my sensor readings:
Here is an excerpt of the TXT file containing my meter readings:
These files require some sophistication to manipulate, but it's easy to import them into Excel or another software program once you get the hang of it.