Click on the "Browse" button and choose the subtitle files to compare from your hard drive, then click on "Compare".

Optionally, a third subtitle "base" file containing the original transcription can be uploaded as well, in case you want to simultaneously compare the differences between files and see how the translation fares as compared to the original.
Should you decide to upload the original transcription, be aware of the fact that the original transcription must have the same timestamps as the original file to compare.