Adding an Existing Field to the Selection Criteria

SOLVED

I want to add a field (Ship Via) to be available in the Selection Criteria Window when running the "Sales Order Printing" Report. I saw in one of the Forums that i have to add an UDF to the Report for it to show up in the Selections. I added that Field as an UDF but it is still not showing up in the Selections. (the field does show up on the report though) 

Can anyone please help me?

See the screenshots below