It seems that Ning has been doing some changes and I am unable to find a way to add a text box to personal pages. As per request of David Olmsted (thanks for that!) I have added a new optional question on your profile that is public. This means that you an use that box to add any information about yourself you would like to share with the group on your personal page.
To do that, click on the 'My page' tab. On your page, on the left click on the manage "my page" hyperlink on the left column, and then click on 'profile'. You will see there the questions you were asked when you joined the network (which are for spam control and the answers are not visible to other users), and now I have added a box with the title "Shared Profile Information". You can add text describing who you are/what you do there, and it will now show up on your profile page. You can position the different elements on your personal page by dragging them up and down.
Let me know if you need any help.