You should probably start with one of the example packages provided.  When you start Simply Fortran, look for example projects that demonstrate PLPlot in the Start panel.  These projects would also be listed if you choose "New Project..." from the Project menu.  These projects would have all the necessary compiler flags configured.

Additionally, in Project Options, you can view the compiler flags or even add them to your projects under the "Compiler Flags" tab. There is even a button, "Package Flags," that will copy flags from package examples into your current project.