If you like to develop add-ins for the PellesC ide you have to download the matching Add-In SDK to your version of Pelles C.
After installation you can find a help file 'bin/help/addin.chm' in the PellesC directory. This help file is also included in the PellesC main help file. You can find it there under 'Integrated Environment –> Add-In SDK'. There is further a directory 'Pelles C Projects/Samples/Add-Ins' in your user directory. You can study these samples to get an idea, how an add-in should look like.
Add-ins are dlls. After creating your add-in you have to copy it to 'PellesC/bin/AddIns' (AddIns64 respectively). Then restart the ide and search for your add-in under 'Tools –> Customize –> Add-Ins'. Check it to use it.
You can find more add-in examples under: