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.
Back at the Ministries and Scheduling page in the "Can't Serve Dates/Times" click "Add". If you are going on vacation or something select the date range or specific dates you cannot serve and the scheduler will not schedule you for those dates. If you know you will not be able to serve every 3rd weekend due to something like a sports event, select "for a repeating pattern" and input that. You can input multiple exclusions.
What if I want to prevent scheduling and want to just ever sign up?
Add unavailable times as per above, select "for a specific service" and add all the weekend services as being unavailable. This should set you to never be scheduled and you can sign up for one of the two sign-up slots for each Mass.