v10.0 can vary (v11.0 for example)
Do a search for your "Microsoft.WebApplication.targets" file and alter the import statement to match.
<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" DefaultTargets="AllTargetsWrapped">
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v10.0\WebApplications\Microsoft.WebApplication.targets" />
<!-- Bunch of Other Stuff -->
<Target Name="AllTargetsWrapped">
<CallTarget Targets="ShowVariables" />
</Target>
<Target Name="ShowVariables" >
<Message Text="MSBuildExtensionsPath = $(MSBuildExtensionsPath)" />
</Target>