Hi Doug,
Support for received messages is not available for the Clickatell service. Clickatell can forward received SMS messages with a HTTP GET or POST request to a publicly available web server. The preferred way to receive SMS messages through Clickatell is to set up a web server with PHP or another scripting language and set up the Clickatell service to forward the messages you this server.
Diafaan SMS Server can receive SMS messages with a GSM or 3G modem connected to the computer.
Regards, Henk