When migrating an Add-in project from VS 2010 to VS 2012, you will need to make a small change to your *.csproj file in VS 2012.
1- Open your Add-in project in VS 2012. Then, unload the project by right-clicking the project folder (in Solution Explorer), and selecting "Unload Project." This will expose the *csproj file.
2- Open the *.csproj file by right clicking it and selecting "Edit *.csproj".
3- In that project file, search for an XML attribute value: "ESRI.ArcGIS.Addins.Targets"
Warning Text="Unable to create .esriAddin; missing ESRI ArcGIS Add-in SDK component(s)." Condition="!Exists('$(MSBuildExtensionsPath)\ESRI\ESRI.ArcGIS.AddIns.targets')" />
4- Change this attribute value to "ESRI.ArcGIS.Addins.11.Targets". Basically, you need to insert "11" somewhere in this attribute value.
5- This XML attribute value appears in 3 different places (as shown above) in your *.csproj file; make the change in each.
6- Search for the tag below and update the target version from 10.1 to 10.2:
6- After making the above changes, Save your project file and close it.
6- Re-load your project by right clicking the project folder and selecting: "Reload Project."
7- Open the Config.esriaddinx file and update the target version from 10.1 to 10.2.
Target name="Desktop" version="10.1" />
8- Build and run your project in debug mode. Upon Building, the Add-in should install correctly, and upon running, you should be able to consume it successfully in ArcMap/ArcCatalog.
Thank you for posting this. I reinstalled my entire dev environment trying to fix this error and of course I still had the error. This fixed it. I am sure this needs added to a help page on ESRI's documentation. There is nothing in there that would help you find out how to do this. Did you find this out from customer support?