Hmm. Have you assigned the modbus device ID correctly? There are some hints in this thread here:
http://infiniteautomation.com/forum/topic/1816/sma-cluster-controller/3
It might be worth reaching out to SMA support as they will be able to help you set up Modbus on the Cluster Controller. There are various 'gotchas' such as firmware version and plant configuration options. And some registers are simply not supported (despite the documentation saying they are).
If it helps, here is the JSON for an Sunny Island inverter attached to a SMA Cluster Controller.
{"dataPoints":[{
"xid":"DP_12341234",
"name":"AC Active Power (All Phases)",
"enabled":true,
"loggingType":"ON_CHANGE",
"intervalLoggingPeriodType":"MINUTES",
"intervalLoggingType":"INSTANT",
"purgeType":"YEARS",
"pointLocator":{
"range":"HOLDING_REGISTER",
"modbusDataType":"FOUR_BYTE_INT_SIGNED",
"writeType":"NOT_SETTABLE",
"additive":0.0,
"bit":0,
"charset":"ASCII",
"multiplier":0.001,
"offset":30775,
"registerCount":0,
"slaveId":4,
"slaveMonitor":false
},
"eventDetectors":[
],
"plotType":"STEP",
"unit":"kW",
"chartColour":"",
"chartRenderer":{
"type":"IMAGE",
"timePeriodType":"MINUTES",
"numberOfPeriods":60
},
"dataSourceXid":"DS_12341234",
"defaultCacheSize":1,
"deviceName":"Sunny Boy 1",
"discardExtremeValues":false,
"discardHighLimit":0.0,
"discardLowLimit":0.0,
"intervalLoggingPeriod":15,
"intervalLoggingSampleWindowSize":0,
"overrideIntervalLoggingSamples":false,
"purgeOverride":true,
"purgePeriod":1,
"readPermission":null,
"setPermission":null,
"textRenderer":{
"type":"ANALOG",
"useUnitAsSuffix":true,
"unit":"kW",
"renderedUnit":"kW",
"format":"#0.###"
},
"tolerance":0.0
}
]}