Personally, I don't like the old IDE's way of handling includes, so I tend to use #include, and just have the include source open in another instance of the editor. The new IDE is probably much better with included files - but I like to work usually with just 1 include file, like a shared library between the game itself and the editor.
Also, the first command in your include file should be END - that way, it'll ignore everything and won't run into a function decleration or subroutine. This allows you to compile the include file to debug it - at least as far as syntax goes.