You've got it all backwards.

If you haven't installed Growl yet on your Mac to handle notifications, then you should do it now. And if you did have it installed, you probably saw it disappear during the last system update numbered 10.6.7, which updated Mail to 4.5. As for now, there is not an official release of Growl for the newest release of Mail (or the previous Mail 4.4), so to get it all running you are going to need to do a little poking around the preference files. The good news is that the only change needed is to simply tell the growl plugin that it is compatible with the newest version of mail.

These instructions assume you know how to use the commandline. Without further ado, here is the instructions:

1. Turn off Mail - otherwise Mail will instantly uninstall Growl
2. Download and Install latest version of GrowlMail from http://growl.info/growlmail
3. Edit GrowlMail Config File
   1. Using a terminal text editor, open the file: /Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info.plist or the alternate per user file: ~/Library/Mail/Bundles/GrowlMail.mailbundle/Contents/Info.plist
   2. Under the key SupportedPluginCompatibilityUUIDs, add to the end of the list the lines:
      <string>9049EF7D-5873-4F54-A447-51D722009310</string>
      <string>1C58722D-AFBD-464E-81BB-0E05C108BE06</string>
4. Turn on Mail - Growl should be working as it did previously.

Note: If you did not find the plist in the proper place then you have probably turned on mail, which will disable the Growl module and move it to 'Bundles (Disabled)' folder. You can either move it to it's correct position or reinstall as per instructions

Good Luck out there.