The current version of ClearScript is not compatible with VS Express 2012, and I think that's a shame for an excellent project like this :)
As far as I can tell it involves very few changes to make it compatible:
V8Update should use msbuild to build the project instead of devenv:
msbuild /p:Configuration="%mode%" /p:Platform="x64" tools\gyp\v8.sln >build.log
Any references to the ATL should be removed since this is not part of VS Express. This shouldn't be too difficult because it seems to only really used in the line:
m_DebugAgentEnabled = Debug::EnableAgent(CW2A(pName), debugPort);
in the V8ContextImpl.cpp file.
Would it be possible for you to implement the changes to make it compatible?