Select the dimension in the note and in the property manager at the bottom you will see
a border section. Just select box and it will give you the appearance of a basic dimension.
depends on your SW version. In SW 2011 you can hi-light the characters then in the property manager near the bottom, there is a section with border. From there you can select the border requirements and the hi-lighted characters will have that border.
After you type "DIAMETER A LOCATED AT" select the dimension from the dwg and it will append your note with the box if its a basic dim.
Both Roy and Tony's methods are two different ways to get it done. Tony's method links an existing dimension to your note. If that dimension exists, this would be the best way to do it, as it will always stay updated.
Roy's method will also work, and I have added a screenshot of how to do that below.