This posting is older than 6 months and can contain outdated information.
The button string you're looking for is "playPause".
There's a complete list of available buttons in the Remote Buddy AppleScript dictionary. You can view that dictionary via the Library in "(Apple)Script Editor.app".
For reference, here's the complete part on scriptremote:
-- dict --
scriptremote v : The Script Remote is a virtual remote control input device in Remote Buddy. It enables you to f.ex. send events from driver software to Remote Buddy and use Remote Buddy as if it would directly support the hardware supported by that driver software.
Sample code:
-- Below line should be executed by the driver software when the play/pause button is pressed
tell application "Remote Buddy" to scriptremote button "playPause" event "press"
-- Below line should be executed by the driver software when the play/pause button is released
tell application "Remote Buddy" to scriptremote button "playPause" event "release"
-- Below line should be executed by the driver software, if it can only execute a script at the time the button is pressed
tell application "Remote Buddy" to scriptremote button "playPause" event "click"
scriptremote
button text : Name of the button to send an event for. The core set of buttons is "left", "right", "plus", "minus", "playPause", "menu". The advanced multimedia button set is "mute", "mmStop", "mmPlay", "mmPause", "mmSkipBackward", "mmSkipForward", "mmFastForward", "mmRewind", "mmVolumePlus", "mmVolumeMinus".
event text : The kind of event. "press" to press the button, "release" to release the button and "click" to quickly press and release the button.
-- / dict --