There's no option to set default through the list. Only "modify" or "delete"
All search URLs work perfectly well in MX5 v.5.3.8.2100. I've copied all of them from MX5 to MX6 using "copy & paste" for each URL, so that there were no typo errors. The Google ones are really long and don't show completely on the screeshot, but the shortest one of the "Real Academia Española" is complete on the screenshot and doesn't work either in MX6.
Thank you very much for you reply, BugSir006!!!