This is a two-step process. I understand that you may have already done these, but I'll write the lot so it benefits others that may read this thread.
In your Contacts App, you will need to add birthdays to each person (or just on the people you want). To do this, you click on the person, then Edit the entry. Scroll down and click on Add field. Click on Birthday, then enter the date of the birthday.
In the Calendar App, you need to activate Birthdays. To do this, you click on Calendars (top left) and ensure that Birthdays is ticked.
Once done, you'll have birthdays automatically showing in your calendar with a little red-wrapped present next to their name. I don't know what's in the present... I think it's an iPad 3. (cross fingers. Lol)
Note, All this is all based on the default Apps that come with the ipad2.