If it is a pension that you have paid into, then you will have an amount of contribution to the plan that can be recovered using the Simplified method. The simplified method cannot be used until you reach minimum retirement age.
The software has a worksheet in it to compute the tax free portion of return of your contributions.
Each year until the year you reached minimum retirement age, the full amount of disability pension distributed should have been taxable.
You will need to verify the amount of contributions to the plan and they may be reported on your 1099R.
Please refer to the following, if applicable:
How to calculate the simplified method portion of taxfree retirement
