To enter RC5 Codes,

Volume Up
1.) Double Click on Button to reveal "Button Properties" Window.
2.) Select Set IR and the Advanced Button.
3.) Select RC5 and enter,
System 19
Command 16
4.) Select OK, OK, OK.


Volume Down
Repeat 1 & 2
3.) Select RC5 and enter,
System 19
Command 17
4.) Select OK, OK, OK.

If those don`t work try a different System Number i.e.17, 18, 20, etc.(System 16 is Default). The Command Number should be the same.

Cheers
Jason

PS. Info here [Link: remotecentral.com]