We ran a hint about this back in 2010, but it's good to remind people how to do this. You want to run two commands in Terminal to get two different UUIDs:
defaults read /Applications/Mail.app/Contents/Info PluginCompatibilityUUID defaults read /System/Library/Frameworks/Message.framework/Resources/Info PluginCompatibilityUUID
19B53E95-0964-4AAB-88F9-6D2F8B7B6037
2183B2CD-BEDF-4AA6-AC18-A1BBED2E3354
Look inside your plug-in's folder, and find the info.plist file, in a Contents sub-folder. Double-click the file to open it with Xcode, or use a text editor. If using Xcode, click the disclosure triangle next to SupportedPluginCompatibilityUUIDs, then click + to add each new UUID.
If you're doing this with a text editor, you want to add the new UUIDs in the end section of the file, which looks like this:
<key>SupportedPluginCompatibilityUUIDs</key> <array> <string>2183B2CD-BEDF-4AA6-AC18-A1BBED2E3354</string> <string>19B53E95-0964-4AAB-88F9-6D2F8B7B6037</string> <string>2DE49D65-B49E-4303-A280-8448872EFE87</string> <string>1146A009-E373-4DB6-AB4D-47E59A7E50FD</string> <string>6E7970A3-E5F1-4C41-A904-B18D3D8FAA7D</string> <string>EF59EC5E-EFCD-4EA7-B617-6C5708397D24</string> <string>4C286C70-7F18-4839-B903-6F2D58FA4C71</string> <string>608CE00F-4576-4CAD-B362-F3CCB7DE8D67</string> </array>

