This is due to a restriction in Excel 2010 where it doesn’t recognise that two Excel files are open. You will need to use the switch windows function.

To link your cells, click into the cell you want to link and then click the ‘link cells’ button (at the end of the row). This will then populate the dialogue box asking you to select the cell you want to link to. At this point click on view (in the excel tool bar) click switch windows, and then select your source file from the drop down list. This will then open your source excel spreadsheet and allow you to select the cell you want to link.