var data={  
   "@ActivNUM":"10525",
   "@MDNUM":null,
   "ClientData":{  
      "@ClientKey":"S10525",
      "ActivEventData":{  
         "@ActivEventNUM":"ACTIV_LOG",
         "@ActivEventRepeatKey":"1",
         "FormData":{  
            "@FormNUM":"TEST_10",
            "IGroupData":[  
               {  
                  "@IGroupNUM":"1_GR",
                  "IData":[  
                     {  
                        "@INUM":"2",
                        "@Data":"#2"
                     },
                     {  
                        "@INUM":"2",
                        "@Data":"FAIL"
                     }
                  ]
               },
               {  
                  "@IGroupNUM":"1_GR",
                  "IData":[  
                     {  
                        "@INUM":"1",
                        "@Data":"#1"
                     },
                     {  
                        "@INUM":"2",
                        "@Data":"SUCCESS"
                     }
                  ]
               }
            ]
         }
      }
   }
}console.log(JSON.parse(data).ClientData.ActivEventData.FormData);//???

console.log(data.ClientData.ActivEventData.FormData.IGroupData[1].IData[1]['@Data']);