You can change the missing data value to whatever you want quite easily. As it says in the Data > Data FAQ section of the MediaLab manual:
Can I change the value used for missing data?
By default, 99 is used as the value for missing data. Because most item types are either open ended or limited to a numeric value of 12, 99 is usually safe. However, if you suspect 99 might be a dangerous value for your particular study you can choose an alternate value. Simply create a text (.txt) file in your experiment folder and name it "missing.txt" and on the first line of that file, enter the alternate value. It must be numeric.
Jason Reed
Empirisoft Software Support Specialist