1.there are some examples of add-ins in Samples folder of the WinAsm Studio full package.
2.all (nearly) add-in projects have source and can be used as examples.
3.to compile add-in you will need WAAddIn.inc for masm or WAAddInFasm.inc for fasm compiler.WAAddIn.inc has some usefull comments.
4.WAAddindlib is lib with some usefull functions to help you write add-in with masm.
to start unpack source of any simple add-in and try to recompile it, then try to use it, then modify and try again. personally, i edit/compile add-in from one WinAsm Studio instance and try it in another (on a case of crash