The challenge will be to align the price series so that you can measure the correlation. If it's daily bars that you are correlating, that task shouldn't be too difficult. But if you are using intraday data, make sure that the dates and times line up on the same row.
You can then use the Excel function "=Correl(series1, series2)" where the series are the ranges of data that you want to correlate with each other. Presumably, this will the the C data from your CSI data source. The function will return the correlation coefficient, which is the number that almost everyone uses when discussing correlation.