These files are each about 50Mb and are the raw data recorded at approximately 1/10th sec, with the extension *.dat. These data form the basis of the entire data set. They are in an archived format and must first have to be converted to ASCII CSV using Campbell Scientific Loggernet software.
If the data is in the TOA5 format, the contents are comma separated ASCII data. You can just do a File\Open in Excel, then save a copy.
If the data is in a binary format like the dataloggers will save onto memory cards, the utility to convert them is called CardConvert. You will find it in the Data section of Loggernet. Output to TOA5 format and name the file to end with a .csv extension. Excel will open that directly.