Generally a GD&T frame attached to a dimension applies to the center of the feature defined by the dimension. This works only when the dimension specifies a feature of size, like the diameter of a cylindrical hole or post has an axis, or the width of a block or slot has a center plane. For example, a positional tolerance on such a dimension applies to the position of the axis or center plane of the feature. Ordinate dimensioning is not suited really for defining a feature of size in this way, so I'm not surprised you can't apply a GD&T frame to it.
I think you do want to apply the parallel tolerance to the top surface, so just point it to the top surface.
EDIT: Roland makes a good point, so I edited accordingly.
Because GDT controls features, not dimensions. In those cases where the frame is attached to a dimension, the dimension is defining the feature.
For some reason GD&T frames don't stick to ordinate dims like they do with regular ones.
Perhaps SW coded it this way because ordinates aren't used in Y14.5 diagrams, although I don't see where they're explicitly excluded from being used with GD&T.
Diatribe aside, you'll have to workaround SW to get what you're after.
Since frames stick to regular dims, add one of these to the drawing, duplicating the specific ordinate dim.
Then add the frame, hovering on the regular dim until it sticks. Adjust as required for aesthetics.
Move the regular dim to a separate layer which is hidden.
Note: When I selected the dim to move to the hidden layer, the GD&T frame highlighted too, but it remained on the drawing.
Part-gdt.zip 128.0 KB