make sure you have automatically import from X checked. Where X equals either/both iTunes and/or Windows media player.
HTC Sync Manager should then find your music.
I got like a billion songs in my iTunes, I just want a few songs on my HTC. The problem when I want to add a folder is that I get a pop-up screen that tells me I have to select a folder on my computer (not on my server?)...
Close the sync manager. Open iTunes via selecting the actual file folder in Windows. Then open your HTC One files with the computer and drag iTunes music into the One's files. It will automatically convert from the .aac to .mp3 but this will take a long time depending on how music will be transferred over. Hope this helps.