Oops! Sounds like you might have deleted the main character image before swapping it. What happens when you delete something from your game you also are deleting all the actions, conditions, and expressions relating to it, as well as all of its instances. That is why the keyboard is no longer an option. It is always best when swapping a sprite to double click on the sprite you want to change then in the edit image window just load the new sprite right over the old one.
To add the keyboard object back to your game just double click on the layout area and insert the keyboard object.
Do you have a backup copy? If not you could go back through the parts of the tutorial that got deleted.
If you are still having problems with it post your .capx, and I will see if I can fix it. <img src="smileys/smiley1.gif" border="0" align="middle" />