The 3900 express comes with an IR remote so there is no pairing it should just work at the box. If you're trying to pair a wifi remote purchased separately, use the IR remote to go in to the settings> remote & devices> pair remote> start pressing and holding pairing button in the battery compartment, release after 30 seconds for a bit and start again until you get a confirmation its paired.
If you don't have a remote to go into the menu, unplug the express from the power, then plug it in and start pressing and holding pairing button, stop when the roku stops bouncing for a bit and start pressing again until you get confirmation its paired.
Edmund
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