I have it figured out (see below). And to help others that may want to use a SIM900 with AT&T I offer the following regarding the AT commands needed. Note; the way your compilier handles imbedded quotation marks may be different than using "CHR(34)."
Print "AT+CMMSINIT"
Print "AT+CMMSCURL="; CHR(34); "mmsc.cingular.com"; CHR(34)
Print "AT+CMMSCID=1"
'my connection is to AT&T in the US. They provide the MMS Gateway as
'"wireless.cingular.com." For the SIM900 the gateway must be a specific
'IP address, Using an online IP and Domain checker I found that the address for
'"wireless.cingular.com" is "066.209.11.32"
Print "AT+CMMSPROTO="; CHR(34); "066.209.11.32" ; ; CHR(34); ",80"
Print "AT+SAPBR=3,1,"; Chr(34); "CONTYPE"; Chr(34); ","; Chr(34); "GPRS"; Chr(34)
Print "AT+SAPBR=3,1,"; Chr(34); "APN"; Chr(34); ","; Chr(34); "wap.cingular"; Chr(34)
'add password and user ID here if required (not required for AT&T MMS)
Print "AT+SAPBR=1,1"
Print "AT+SAPBR=2,1"
Print "AT+CMMSEDIT=1"
Print "AT+CMMSDOWN="; CHR(34); "TEXT"; CHR(34); ",6,5000" 'text of 6 bytes and
Print"3280wa" 'send MMS message to Sim900
Print "AT+CMMSRECP="; CHR(34); phone ; CHR(34) 'add first recipient,
Print "AT+CMMSRECP="; CHR(34); email ; CHR(34) 'add second recipient,
Print "AT+CMMSVIEW"
Print "AT+CMMSSEND" 'send MMS to AT&T
Print "AT+CMMSEDIT=0" 'Exit edit mode and clear MMS buffer
Print "CMMSTERM" 'Exit the MMS function