This element allows you to specify a set of target frameworks that will be used as fallback when restoring packages. In project.json it was represented as an imports statement inside the frameworks or tools node. The semantics of <PackageTargetFallback> are the same as imports in project.json.
This element is an MSBuild property. That means that it needs to be placed inside <PropertyGroup> element, either a new one or an existing one in your project file:
<PropertyGroup>
<PackageTargetFallback>$(PackageTargetFallback);portable-net45+win8+wp8+wpa81;</PackageTargetFallback>
</PropertyGroup>