how to add alt text field in image field?

Go to admin/content/types, click "Manage fields" on the node type that contains the ImageField, then click "edit" on the ImageField. Under the ImageField options, there is a checkbox for "Enable description".

Oh right, sorry I meant enable the option for "Enable Alternative Text", not description. Oddly the "description" field doesn't do ANYTHING when used with ImageFields. You need to add custom theming in order for it to have an effect.

Further, the text you see on mouse over is not the Alt text at all, it's the "Title". You should enable the title field if you want to have mouse over text. Note that IE mistakenly uses Alt text for mouse over, but only if a title is not provided (then it use it, just like all other browsers).

So in short there are 3 fields you can enabled: title, alt, and description. Description doesn't do anything and I recommend you don't use it (especially since it's been removed from the Drupal 7 port). Alt is for the text used by screen readers. Title is the text used on mouse over.

Is there a way to force the alt text to be filled in if an image has been uploaded?

Edit:
Sorry, found a thread here.
http://drupal.org/node/347070