Can you download the code from https://msbuildextensionpack.codeplex.com/SourceControl/latest and then add the assemblies from $Solutions\Main\Sql2012\AssemblyReferences to the same location you are loading MSBuild.ExtensionPack.Sql2012.dll from and let me know if it resolves it.
If it does not, please send your full error message and how you are using the task
Thanks
Mike
If it does not, please send your full error message and how you are using the task
Thanks
Mike