Thank you for your inquiry.

I assume you created the arc by adding a circle cutout to the polygon. However, only the actual outline of the polygon is counted as shape when you merge it with pad, and cutout is not a part of the outline of polygon, rather a child object of the polygon.

Thus, you need to create the arc as a part of the outline of the polygon.

Since you already have the polygon and cutout shapes, the easiest way to do this would be to convert those to line shapes, connect those shapes and re-convert to a single polygon.

Please select both the polygon and cutout shapes, right click and click "Convert to line". (Click "OK" to message dialog)

Select the lines and arcs, right click and click "Split shape".

Now that the lines and arcs are split at crossing points, select and delete shapes you don't need.

Select the remained shapes, right click and click "Convert to Filled object" => "Convert to Filled polygon".

Then you can merge the polygon with the pad.

If you have anything unclear, please let me know.

Thank you very much.