If I understand correctly, the current form of what was called UDPBD (UDP Block Device) has been realized in OPL as the more standard NBD (Network Block Device) which is a linux compatible network protocol.
As for how to use it, you would want to go here:
https://github.com/ps2homebrew/Open-PS2-Loader
and read the section under "NBD Server" (the server is integrated into OPL). Under that is some recommended clients for your computer.
That said, it seems to say that NBD is currently only supported as a way to populate a PS2 HDD from your computer (add games from a computer to a PS2 Phat HDD so that the games can be played from internal HDD - not streaming over the network).