The problem may be the drivers for the Prolific PL2303 based USB to serial adapter. Saw this on the MacBS2 site:
'These adapters have the advantage that they are usually very inexpensive. But the manufacturer's drivers did not appear to support setting and clearing of the Tx line, which is required by the STAMP programming protocol.
However, there is an Open Source driver for interfaces based on the PL2303 chipset that does work (I tested it on my PowerPC based PowerBook; I don't know if it'll work on Intel based machines). Download the driver from its SourceForge page. Thanks to Guy Umbright for the heads up on this. '
The SourceForge driver is here http://sourceforge.net/projects/osx-pl2303
'These adapters have the advantage that they are usually very inexpensive. But the manufacturer's drivers did not appear to support setting and clearing of the Tx line, which is required by the STAMP programming protocol.
However, there is an Open Source driver for interfaces based on the PL2303 chipset that does work (I tested it on my PowerPC based PowerBook; I don't know if it'll work on Intel based machines). Download the driver from its SourceForge page. Thanks to Guy Umbright for the heads up on this. '
The SourceForge driver is here http://sourceforge.net/projects/osx-pl2303