You can't reference this in your pcl - it isn't defined in any current portable subset.
If you need to use it, you can reference it in some of your Ui projects - eg in your setup class - or for reuse you can build a plugin to reference it for some platforms. However there are some modern platforms which simply don't support it - WindowsPhone (and perhaps windows store too?)
For more on injecting platform specific code into pcl core libraries, see http://slodge.blogspot.co.uk/2013/06/n31-injection-platform-specific.html