There are various documents at the NI Developer Zone that explain how to log data in a FP-20xx controller.
These examples and tutorials explain in detail how to do this.
In general, once you have got the value from the FP I/O Module in the LV Program, you can use the File I/O VIs to save this information to disk.
Please refer to the NI Developer Zone page
URL: http://zone.ni.com/zone/jsp/zone.jsp
and use the search engine (type the words "fp" and "log") to get the documents that explain how to log data in an FP-20xx.
The URL to these documents is shown below:
http://search.ni.com/query.html?col=alldocs&layout=Zone&qp=section%3Adevzone&qt=fp+log
I hope this helps.