If you are receiving Social Security Disability, the best way to model the income in My Plan is to set up a pension.
STEP 1: Head on over to My Plan > Income > Pensions
STEP 2: Press Add a Pension +
STEP 3: Enter a descriptive name, Social Security Disability, for example
STEP 4: Enter your monthly benefit
STEP 5: Enter the start and stop ages
STEP 6: Adjust the optimistic and pessimistic growth rates to match your Social Security COLA assumptions
STEP 7: Select the appropriate tax treatment
STEP 8: Save