QGIS plugin is still in development.
You can download a functioning sampling script for doing random stratified sampling from the "script" folder, or by clicking here for the latest version from master.
Main dependencies:
QGIS (2.0.1 or newer)
Python (2.7.5 or newer, but not Python 3)
GDAL (1.10.0 or newer)
Python dependencies:
numpy>=1.7.0
gdal>=1.10.0
docopt>=0.6.0
If you have QGIS already installed on your system, you most likely already have everything but the docopt module installed. Luckily, docopt is easily available from the Python Package Index (pip). If you don't have or don't want to use pip, docopt is very easy to install as it is just one single script file. Simply download the docopt.py script from here and place it next to the Python script you're running, or somewhere in your PYTHONPATH.