Inside Tomboy, I'm running the following: Mono.Addins.AddinManager.Initialize ("~/.tomboy"); Mono.Addins.AddinManager.Registry.Rebuild (null); I have the following file inside ~/.tomboy/addins/Tomboy.addins: /home/boyd/stage/tomboy/lib/tomboy/addins I have the following installed into /home/boyd/stage/tomboy/lib/tomboy/addins: Tasks.dll ...which has the following code: [assembly:Addin (Id="TasksAddin", Version="1.0")] [assembly:AddinDependency ("Tomboy", "0.7")] [Extension] public class TasksAddin : ITomboyAddin { public void Startup () { Logger.Debug ("TasksAddin.Startup ()"); } public void Shutdown () { Logger.Debug ("TasksAddin.Shutdown ()"); } } But, I get the following output at startup: [INFO]: Initializing Mono.Addins [DEBUG]: OnAddinLoaded: Tomboy Checking: /home/boyd/.tomboy/addins Checking: /home/boyd/stage/tomboy/lib/tomboy/addins Checking: /data/boyd/projects/tomboy/Tomboy Folders checked (20 ms) Looking for addins Checking: /home/boyd/.tomboy/addins Checking: /home/boyd/stage/tomboy/lib/tomboy/addins Checking: /data/boyd/projects/tomboy/Tomboy Scanning file: /home/boyd/stage/tomboy/lib/tomboy/addins/Tasks.dll Scanning file: /data/boyd/projects/tomboy/Tomboy/Tomboy.exe Folders scan completed (140 ms) Generating add-in extension maps Addin relation map generated. Addins Updated: 1 Extension points: 0 Extensions: 0 Extension nodes: 0 Node sets: 0 Add-in relations analyzed (21 ms) [DEBUG]: Addins: 0 It's picking up the AddinRoot ("Tomboy"), but not my TasksAddin.