PERSON1229 Not all version 2 licenses were eligible for free upgrades to MacDust 3. Any license purchased during or after February 2008 (to the best of my ability, I might have missed some) were added to the automatic upgrade list.
If you have your MacDust 2 license installed, an Upgrade tab should be available on the opening nag screen, this tab will allow you to connect to the internet and check for your upgrade license.
If you believe your license was eligible for the free upgrade and the online check did not return anything, you are more than welcome to email me. My email address is on the About window for the app.
(Version 3.01)