Where do you want to add it? To the homescreen or the the dockbar?
I am also assuming that you are using an HTC Sense based ROM, and are using the HTC Sense mail app. If you are using a different ROM and the normal AOSP email app, then on Launcher Pro you can the option in preferences just like for Gmail.
For a Sense based ROM with the Sense mail app using Launcher pro, for the homescreen, Long press and you get the "Add to Home screen" option. Go to "Shortcuts" and scroll down to "Mail Inbox". That shortcut will have the number of unread emails.
You can do the same thing for the dock. The only thing is, the icon is the HTC icon, and the unread count doesn't work with the LP icons or custom ones.
Good luck.