You have to put these two lines:
...into one of the .decls files in your userlibs folder. An easy way is just to create a "user32.decls" file, copy the two lines in, and save it. If you already happen to have one, make sure the "lib "user32.dll"" line is only entered once (in any of the decls files).
lib "user32.dll" SendMessage%(hWnd%,Msg%,wParam%,lParam%):"SendMessageA"
...into one of the .decls files in your userlibs folder. An easy way is just to create a "user32.decls" file, copy the two lines in, and save it. If you already happen to have one, make sure the "lib "user32.dll"" line is only entered once (in any of the decls files).