It seems that mask support was not available in Origin 7.5. You will need to use the LabTalk <n> notation for now. We have already added this support in next Origin version which is not yet released.
// LT notation data1_b<2>=1 to mask a cell string strLT; strLT.Format("%s<%d>=1", strDatasetName, nIndexToMask); LT_Execute(strLT);