Had you seen this, carreragt1990:
The Death of Hotmail for Mail.app users and a new solution
My hotmail account, the one that works with HTTPMail, is much older than 5 yrs, I can't say exactly when things changed but the newer accounts needed to be upgraded before the plugin worked.
Some recommend Gmail over Hotmail and my Gmail account mail comes to my Mail.app inbox too, so there are choices ... other than this, I can't say why you're having problems ... I recall I needed a few tries to get it right when I was trying to enable the plugin ...
Do you have the HTTPMail folder in:
YourUserAccount ~> Library ~> Mail