I am using a Mac. I am unable to click on the Rename Files box and below I can see that it says Renaming is disabled.
In the import window, please make sure all the panels are expanded on the right.
Also please select the images from the source, then you should be able to check the box ' Rename Files"
Please let us know if it helps.
Make sure you are doing a "copy" import, not an "add" import.