hey goodwin,
you can define the fields you want to display on this mini registration directly in your easysocial backend.
go to profile types -> then click on the profile type you're using. from here go to the tab "custom fields" which shows all the fields associated with that profile type.

now taking birthday (that you want to add) as an example: click on the birthday field, this opens a new box at the right side with some options for this field. click on the "view" tab there and at the bottom you find an option called "Appear in Mini Registration" - set this to YES and that's it..
http://screencast.com/t/5ogy7uJA

the same way works for other fields as well. hope that helps and have a nice day