most off brand dvd players won't be in the urc database.

for discrete codes, you're probably out of luck there too.

there is a possible work around.

see if the play command turns the player on.  if that doesn't work, try the eject command.   if you can get it to turn on, then you can built a macro using those codes, followed by a delay that's long enough for the player to accept the power toggle code.

so:

play
delay x seconds
power

would turn turn the unit off.

As far as having trouble learning the codes, try using the Learn Twice option in the URC window.  Try holding the remote at different angles or distances from the URC remote when learning.