Hi Olwyn,
My first test with your project was to simply install the compiled Add-In, CSTutorial.esriAddIn found in the CSTutorial\bin\Debug folder. I copied it to a temp folder and double-clicked on it to launch the Add-In installer. It installed successfully, the toolbar with button showed on ArcMap startup, and clicking it displayed the "Hello World" dialog.
So there is nothing wrong with your project or Add-In directly, I believe. Likely either a conflict with an older version of the Add-In, or some other system issue.
Have you tried clearing the assembly cache and Add-Ins folders?
Try this:
-Using the Add-In Manager, delete the tutorial Add-In.
-Close ArcMap
-Delete everything in: C:\Users\[USER]\AppData\Local\ESRI\Desktop10.0\AssemblyCache
-Check the Add-Ins folder to ensure the tuturial Add-In was deleted:
C:\Users\[USER]\Documents\ArcGIS\AddIns\Desktop10.0
(if still there, delete it)
-Do a manual install of the Add-In as I did, and see if it works.