I found the answer. You have to go to the dialog that Hilary mentioned, but also do these additional steps:
1. Under MyFields -> data fields, find the repeating section and click on it.
2. There will be a downward arrow on the right. Click on it.
3. Select "Add another list item below/above" depending on where you want the default repeating item.
(USEFUL: Instead of clicking the down arrow hit the "insert" key to quickly add more default rows.)
4. Set the default text if necessary for any of the fields in the new input row.
5. Click "OK" when ready.
6. Profit.
Example screenshot below.
Paraphrased from this article: http://blogs.msdn.com/b/infopath/archive/2006/07/12/don-t-settle-for-just-one-adding-multiple-default-rows.aspx
One more question though...is it possible to divide the repeating section into sub-sections so I can place label headers in between the input rows like you would a survey?