I don't know if this a bug but I thought I should post my findings for fellow fans of the "Forms" mode. You CAN'T enter your IP Pin in the "1040 Worksheet" when in "Forms" mode. The box is there to the right of your signature but TT won't let you enter it. (You also can't enter your email address.)
In "step-by-step" mode it's easy. You click on the "other tax situations" button, scroll down to the "other return info" section, and click the "update" button.