I found solution for my keyboard... When it pops out to put PIN just tipe 4 digit number and click connect and then press the same 4 digit PIN code on your bluetooth keyboard and press enter/return
Hope you found it useful. Please let me know.
I had to use an 8-digit PIN in order to get this method to work. And I had to try it about 50 times. But finally the stupid bluetooth keyboard connected to my PC. What awful technology this Bluetooth stuff is -- how did it ever become an industry standard?
Keyboard is an old (from 2005?) Xboard bluetooth trackball keyboard, manufactured by Wireless Engineering Inc.