This is how a mask works.  The areas you want to keep should be assigned a 1 and the areas you want to change should have a value of 0.  If the mask looks correct, go to Basic Tools ->Masking ->Apply Mask to mask the areas of interest.  You will need to set a value for areas of '0' in the mask.   If your only need is to exclude these areas when processing data, most of ENVI's file selection dialogs allow you to select a mask band so that you do not need to apply the mask to the data.  The difference is when you apply the mask, you have to give these areas of 0 in the mask some sort of value.  When processing, ENVI uses all data and it can affect the results.  When using the file selection and selecting a mask band, these areas are actually excluded from processing.