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.