Are your sure your Excel file is structured properly, with a tab titled 'NiiStat'? I think it may not like the field name for age, please try to remove characters like _ and () to simplify the title.
Roger


"You will get an error if your Excel file does not have a worksheet named "NiiStat" (case sensitive)
Error using nii_xls2mat (line 97)
Invalid field name: 'age_(yrs)'.
Error in nii_read_design (line 18)
[dMat, nuisance] = nii_xls2mat(xlsname , worksheetname,'', true);
Error in NiiStat (line 87)"