How is the Van der Pol differential equation implemented in PLECS C-Script?
Before you endeavor to write a C-Script, you might consider using built-in blocks. An ODE (or as in this case: a set of first order ODEs) can easily be implemented using Integrator blocks along with a handful of function blocks.
If you have to write a C-Script, please read the chapter C-Scripts in the documentation first. It contains a set of examples including a basic integrator. Extending this to a set of ODEs should be straightforward.
If you need help, please share your effort with the community and point out where you are encountering problems.