How does one adjust the equivalent of the barrel adjuster on the XTR RD-M9000 rear derailleur?
Page 19 of the linked Dealer Manual shows the barrel is in the shifter, up on the bars.  



From https://si.shimano.com/pdfs/dm/DM-RD0004-09-ENG.pdf

If yours doesn't have one, are you using a different/wrong or off-brand shifter?  You will need to fit an in-line barrel adjuster like this: