For you to be able to use the Correlation tool you need to load the Excel Analysis ToolPak. You can also calculate the r value in Excel in the more traditional method but with the help of the automatic calculations from the program. If your data is saved as such you can use one of the easiest and most general options to import your file to R.
Click File from the tab list.
Note the value of R-squared on the graph. If youre not sure how to load the toolpak heres a summary of how to load the Analysis ToolPak. Using the R-squared coefficient calculation to estimate fit. To understand r-square more read regression analysis in excel.