it says to "Confirm that the add-in is deployed to the well known folder where the Desktop applications will access it. Typically this is already done because add-ins created with the ESRI Visual Studio integrated development environment (IDE) template are automatically deployed each time they are built."
Well, I have created a Visual Studio Add-in Application for ArcCatalog, and when I build it or debug it, the .esriAddIn file does not get copied to the AddIns folder in my user\documents\ArcGIS folder.
I have tried to add a Post-build event in Visual Studio to copy that file but it seems that the file is not immediately available after build and so the copy fails.
If I manually copy the .esriAddIn and manually start ArcCatalog, the add-in works fine but to debug it I have to manually attach Visual Studio to the ArcCatalog process everytime, which is very time consuming.
by defult its not neccessary to do this, if you have created an addIn-Project in Visual Studio. Maybe, you can go back, make the options to default, and create a new addIn project importing your old developments.
Yes, you can make 4.0 work but ESRI does not deliver it and you have to do a bit of configuration. Search the forums for this because it has been discussed.
Here is a few of the posts I quickly found. There are many more I remember seeing some time ago, including posts which describe how you modify the config file in order to make the 4.0 framework work.
My recommendation is to stick with 3.5 and move to 4.0 only after ESRI supports it.