How do I get the icon for my e-mail back on my menubar?

Are you referring to the Dock or the Menu Bar?

The Menu Bar is the thin bar that runs across the top of your screen. It has the Apple logo at the far left, and next to that, the name of the currently active app. There is usually no Mail icon there.

The Dock is the thing that has a lot of icons, for apps and folders usually; it is either on the bottom or the sides of your screen.

If you are referring to the Dock, go to Applications, and drag the Mail program to the Dock.

Mail Unread Menu ➚ does this, but the reviews ➚ on MacUpdate seem to imply some instability.