Here is a little snippet from the ASPECT help file that explains why this isn't working:
"Only keycodes which have an actual keyboard equivalent can be used in a key-sending command. The mouse-related keycodes, for example, cannot be sent. Likewise, you cannot use the keyget command to see a mouse button event. However, you can use the mouse keycodes in a command such as keystate to determine the state of a mouse button."
If you are setting focus to a dialog of similar UI, you can likely send a space instead and activate the button (that's what I assume you are doing) in that manner.