Ah yes, this was posted on the "Writing Your Own Software" section didn't see that!
To answer your question:
short ps3000_set_trigger (
short auto_trigger_ms )
the source which is a short with enumerated types so :
PS3000_CHANNEL_A (0), PS3000_CHANNEL_B (1),
PS3000_CHANNEL_C (2), PS3000_CHANNEL_D (3),
PS3000_EXTERNAL(4) or PS3000_NONE(5)
So external would be a value of 4 if you wanted to manually type it in.
Hope this helps.