If you have do not have permissions to edit the form, you will not find where to
do so. Sometimes a form may be customized to show certain fields. Other times,
the field itself may be configured to not show on the New form, but to show
elsewhere. With proper permissions, however, you would be able to customize the
form to expose the field. This may require Designer or InfoPath depending, but
if you don't know how or don't have the permissions, I would elevate the issue
to the appropriate people in your organization. Otherwise, there are plenty of
books and blog posts that address customizing forms.