Did you set the OnLayoutKeystroke script trigger for that layout?
The script is simply:
Go to Field [ ChildTable::YourField ]
If [ Code ( Get ( TriggerKeystroke ) ) = 31 ]
Go to Portal Row [ Next ]
Else IF [ Code ( Get ( TriggerKeystroke ) ) = 29 ]
Go to Portal Row [ Previous ]
P.S.: I do not know where did you get those values for up & down arrows. See:
Thanks - worked an absolute treat - once I realised that I had to use script trigger at layout level rather than field level. Sorry that it took so long so to get back to you.