I have a similar problem on my Nexus running Android 5.
When I try to pair the pen, the following dialog is shown without any way of entering the passcode:
"Bluetooth pairing request
Device
NWP-F110
Type the pairing code then press Return or Enter
Pairing grants access to your contacts and call history when connected.
Cancel Pair"
No keyboard shows up so I'm unable to enter the passcode. No matter what I click, I'm returned to the step 2 screen in the Neonote app, where it continues to search for the pen. In my Bluetooth settings on the phone I see the pen as paired, but still have no way of modifying the pairing.
If I simply click on "Pair", the pen beeps again and continues to blink blue, but nothing else happens - the app just continues to search.
If I repeat the whole thing the app hangs at step 2 indefinitely - that is, unless I forget the pen in the phone's Bluetooth settings and then start over again.
How can I get past this step?