How to Create Dynamic Fields in a Meta Box?

You are looking for Repeatable Fields.

Here in WPSE, it was covered in the following Q&A:
Custom Meta Boxes: Store two values in one repeatable field

A Gist:
https://gist.github.com/2057532

Class helpers for building the plugin options page:

• Option Panel Class ^{(from Bainternet, one of the house moderators)}
• WPAlchemy_MetaBox PHP class