Writing file to SD Card

To save a file to the SD card it best practice to use the following environment variable(EV). ( 'Android.OS.Environment.ExternalStorageDirectory')

However, when I attempt to access this variable I get the message "The type or namespace 'OS' does not exist in the namespace 'SampleApp.Android'.

What is the best way to get the write location of the SD card. I have created a PCL project named SampleApp and I am attempting to access this EV in the android project.