I'm able to slide the extension nipple onto stub out nipple but I cannot get it close enough to thread ont the adapter. Its as if there's something keeping it from nesting properly. I have the adapter close to the wall so as to avoid a gap but I'm stuck. Any ideas?

Please explain what a "extension nipple" is. How does it connect to the stubout?

I cannot get it close enough to thread ont the adapter. close enough to what? I'm assuming that you have a screw on spout and not a slip on spout, ( which would install much easier). Can you furnish more details or a picture? Back to you, Tom

Thank you for reaching out. I've inserted a link to the instructions here:
http://www.danco.com/files/multifile/UniversalTubSpoutHiddenDiverter-Chrome-10315-Instructions.pdf
I have a smooth stub out and I believe a screw on spout but this new one won't assemble to save my life. I've slipped the Adapter (A) onto the stub out with the threads near the wall. Then I added the O-ring, followed by the Extender (C). When I have it in my hand, all the pieces go together fine. When I stall it, the extender can't get close enough to the threads to screw onto the adapter. There has to be something obvious I'm missing...

What I can't figure out is why you insist on doing this the hard way when a slip on spout ,(see image) would be so much easier. Is there a reason? Back to you. Tom