Vipin,
For your reference, you should be able to find help on the VI in question by clicking on help->Show Context Help. This will give you help for the element on your block diagram that the mouse is hovering over. File path is the path/name of the file you would like to write to. If it is not wired, LabVIEW will prompt you for this. Append to File is used to specify whether you want to write over any existing data in the specified file, or append to the end of it.
For your case, I would actually recommend that you don't use the Write Waveforms to File.vi at all. If you open it's block diagram, you will see that it will both open and close the file specified. However, it is MUCH more efficient to open a file once, write to it multiple times, then close when you are done. In the attached example, I basically opened the block diagram for the Write Waveforms to File.vi, and placed it's sub-VI's into your VI. If you look at what I've done, you'll see that I open the file to write to at the very beginning of the program. I then configure the acquisition. Next, in the while loop I read data and write it to the file we opened at the beginning of the program. At this point the program will continue to acquire data and write it to the file. Finally, after the acquisition is stopped, the file is closed.
The error you were receiving is likely do to the fact that since you had the file path unwired, you were prompted for file. While this occurs, you are unable to read data out of the buffer from your acquisition. As a result, your DAQ device continues to write data to the buffer, and eventually overflows it, hence the error you received. Try running the attached example and see if this does what you would like. Also, I strongly encourage you to use the context help if you have questions about the VI's that ship with LabVIEW. I've been using LabVIEW for a few years, and I still find it to be very helpful.
