Yesterdy we released MAMP & MAMP PRO Version 1.9.1 (download page). We've fixed a bug in the Japanese localization that the Hosts pane didn't show up.
This update is already available on our download page, don't hesitate to report any other bug you may find to our bugbase so we can fix it as quickly as possible.
All MAMP PRO updates in the current major version (1.x) are free of charge. If you want to update MAMP PRO from e.g. 1.7.2 to 1.9.1 just use the serial number you already got.