Edit links in Excel on Mac

[blackjm75]

Working on Excel for mac 2011 and the file that I'm trying to work on is a .xlsx file. When trying to EDIT LINKS > CHANGE SOURCE all of the other workbooks that end .xlsx are greyed out and cannot be selected. The only spreadsheets that are not greyed out are .xls files. We have a lot of spreadsheets that are linked with others and obviously ideally I don't want to save all my excel spreadsheets at the old format (.xls).

Does anyone have an idea why this might be happening and how to rectify it?

[romperstomper]

Is your copy of Office fully patched up to date?

[blackjm75]

Yes, I just double checked and had 1 update to install but installing this still left the problem.