Save the setup program to you computer.
Run it and follow the installation instructions.
Start PhytoSim and create an account (if you don't have one already).
Confirm your account by clicking the confirmation link in the email that was sent to your email address.
Login to PhytoSim.
In the module manager install the modules that you are interested in.
Click on a module and press the 'Install module' button on the tool bar.
Have a look at some of the examples located in the PhytoSim/Examples folder.