A problem was encountered creating the sub project…. Microsoft.Lightswitch.Toolkit.targets” was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.[/i]

After installing the LightSwitch Extensibility Toolkit. you might get this issue when you try to create  LightSwitch

Extensibility  project .  

Here is the  Solution for this

  Copy the Microsoft.LightSwitch.Toolkit.targets file into %Program Files (x86)%\MSBuild\Microsoft\VisualStudio\LightSwitch\v1.0
This will fix this issue . For detailed installation of LightSwitch Extensibility Toolkit follow the below steps
Steps for LightSwitch Extensibility Toolkit for Visual Studio
Visual Studio 11 Beta SDK is required to use the LightSwitch Extensibility Toolkit for Visual Studio 11.
You can download the Visual Studio 11 Beta SDK from - http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=28990

To install the LightSwitch Extensibility Toolkit:
1. Double click on Microsoft.LightSwitch.Toolkit.vsix present in this zip file.
2. Copy the Microsoft.LightSwitch.Toolkit.targets file from this zip into:
   - For 32-bit systems: %ProgramFiles%\MSBuild\Microsoft\VisualStudio\LightSwitch\v2.0
   - For 64-bit systems: %ProgramFiles(x86)%\MSBuild\Microsoft\VisualStudio\LightSwitch\v2.0

Launch Visual Studio and project templates for LightSwitch Extension Library will be available under the LightSwitch/Extensibility node in the New Project Dialog.