The Flags column is the data qualifier column for your data set. Select the specific flags that you want to indicate non-detect data. For example, if you want a U flag to indicate a non-detect then select U from the list and VSP will turn on (check the box of) the Non-detect column for those results that contain a U flag. You can select multiple flags, each of which will indicate a non-detect.
Note that when a result is marked as a non-detect, the number in the Value column is used as the detection limit for that result.