It sounds as if you've built your own toolwindow and want it to function similar to the Solution and Server Explorer toolwindiows. In that you'd like to launch an editor (or designer) when you dlbclick on an item in your tree control.
With the Solution and Server Explorer toolwindows, the dblclick action is processed by the underlying UIHierarchy interface that is fed to the VsUIHierarchyWindow. Typically, the dlbclick will launch an editor (or designer) by way of calling one of the methods off the IVsUIShellOpenDocument interface. OpenDocumentViaProject, or OpenSpecificEditor, etc. These methods requirean IVsUIHierarchy pointer, and an itemID, as the IDE will attempt to register these documents in the Running Document Table, and use the IVsUIHierarchy and itemID to tie the opened document to a specific project/hierarchy.
You could implement a custom IVsHierarchy/IVsUIHierarchy interface on your tree based toolwindow, so that you could leverage that IVsUIShellOpenDocument interface, or if the items in your toolwindow represent actual files, you could potentially use the DTE.ItemOperations.Open to open a specific file in the editor.
But in all likelihood, you'll probably want to implement a custom heirarchy for your treecontrol based toolwindow. I have a sample uploaded to the gotdotnet codegallery called HierAnarchy that implements a very minimal IVsUIHierarchy interface, but it's actually wired into a VsUIHierarchyWindow based toolwindow. You might want to give it a look over, as it could provide you with some ideas.
Sincerely,