How to set your Mass preferences and exclusions
Login to MSP and go to your profile
Click "Add" to drop down the Mass selections. When you have input your preferences check off "Schedule me ONLY at these times" to serve only those Masses. If you don't do this the scheduler will assume you are open to serving other Masses as a fill in and you might get scheduled for something you don't want to serve or can't serve.
Select the Masses you prefer. You can only select one at a time and you may select multiple Masses. The scheduler will load balance you across your preferred Masses in addition to other factors such as how many times you've served.