The mail indicator in the top panel won't be visible if no programs are using it. To see whether you have any programs using the mail indicator you'll need to use dconf-editor. If you don't have it installed, you'll need to install it first by opening the terminal and typing
sudo apt-get install dconf-editor
After which you can run it by pressing ALT+F2 and entering dconf-editor.
When it's open, you'll have to find the messages indicator. I'm using Ubuntu 14.04, so on my system it's in com.canonical.indicator.messages. If it's empty, you can then manually add programs to it, like ['thunderbird.desktop','unity-mail.desktop'], etc.
Make sure to restart the unity panel after you're done by pressing ALT+F2 and typing
restart unity-panel-service
The mail icon should then appear on the top panel.
However, I've just tried installing Unity Mail myself, and the notifications don't work, neither on the top panel nor on the dashboard on the left. This appears to be due to an unsolved bug within the program.
Good luck, and let me know if you find a program that works!