There is a wonderful and easy workaround for adding alarms to iCal events entered in the integrated Bento iCal Library.

Recently I learned about a somewhat hidden (and rarely talked about) feature in the iCal application: in the general preferences just click the checkbox entitled:
“Add a default alarm to all new events…” and specify other detailed options. Once you’ve done that, you never have to open iCal again. Whenever you create a new iCal event from within Bento, an alarm will be added automatically and transparently.

This takes care of one of the many strange and basic shortcomings of Bento.

Have fun!