This message was longer than the maximum permitted -- so here's the end of it, cut and pasted...
On the plus side, the Confessions, as I recall, includes an index -- so, if you can locate a copy, it should be easy to look for a Machen reference.
On the minus side (checking on Amazon) the Confessions seems to be out of print. On UK Amazon, the cheapest copy is £25. Alternatively, you may be able to find the book in a library.
After I'd checked Amazon, I closed it, but mistakenly closed the Internet connection rather than just the Amazon tab. In doing so, I lost my original version of this message. It felt, to me, like the curse of Crowley striking. I don't think that I'll look him up on Amazon again, let alone read any of his stuff. Some rocks are better left unturned. I do not recommend looking into his Confessions, or any other Crowley book. It may be safe for you to do so, but please don't explore Crowley too far.