jay2012:
Jack,
here is the product,
XBee Pro 900 XSC RPSMA - WRL-09087 - SparkFun Electronicsok,this is a special product by Digi, and they do not make this anymore.
Actually that is an S3B part, and while that particular part number (XBP09-XCSIT-009) doesn't appear to be current (I don't find it on Digi's web sire), I think these are equivalent.
You say you're trying to use API mode, that could be an issue, according to the product manual, this series does AT mode only. There is a binary command mode, but it appears limited to reading and writing module parameters, not sending or receiving the actual live data. Not sure what doc you may be following to do the API interface, but whatever it is, I think that it is inappropriate for this particular XBee series.
I don't recall seeing any discussion on this forum related to S3 modules and I haven't worked with them at all, but as always, GIYF.