I suggest putting this post up in the Bento forum to get better responses. This is the Filemaker Bug form and your issue doesn't seem to have anything to do with Filemaker.
I didn't see your post on the Bento forum, so if you are still having difficulty, please let me know what occurs when you try to enter the License Key? If the "Use License Key" button is not highlighted, then make sure you either entered the License Key correctly, or make sure you are using a Bento 3 License Key. If you try to install with a Bento 2 License Key, it will not work.
iCal Events should display all Events in your iCal Calendar for the previous two years until two years from now.