You can define the user DRO # in a screen designer, like Machscreen or Screen4.
When you're program uses User DRO's, then typically, you are the one that creates the DRO's.
In your example, you'd need to add UserDRO 1151 to the screenset you're using, unless you're using a screenset that someone else already added.
The default mach3 screenset contains no UserDRO's.
Hope this makes sense.