Thanks for your note to Feminist.com and for your commitment to ensuring that women are included appropriately in history.
It is true that women's contribution in our telling of history is minimized. It amazes me that the few women we learn about are Betsy Ross, who was basically a seamstress, and Rosa Parks -- their contributions are important, but there is so much more that women have contributed. The National Women's History Project does a good job of attempting to write women back into history, as do many of the feminist historians that exist.
As for making a case, I think that you should have some examples, which the above should be able to provide you with -- and then use these examples to prove how much more we must be missing out on. I hope that helps.