If you are just using the add-in for yourself, you can simply go to View > Customize > Menus and Toolbars. Expand Add-Ins and select your add-in. In the icon section, click the "built in icon" radio button. Then you can select a JMP icon to appear in the menu item for your add-in.
If you are distributing this add-in for others to use, it is a lot more complicated because these settings need to be reflected in the .jmpaddin archive file.
1) Go to View > Add-Ins... Click on your add-in and click on the home folder link to open Windows Explorer (if
using Windows).
2) Open the addin.jmpcust file in a text editor such as Notepad.
3) Change the line that begins with "<jm:icon" to look like the following (replace "JMP" with the name of your
icon):
<jm:icon type="builtin">JMP</jm:icon>
4) Save the addin.jmpcust file.
5) Open your add-in using Add-In Builder.
6) Under the Additional Files tab, add the addin.jmpcust file that you saved previously. This will overwrite the
addin.jmpcust file that is automatically created by the Add-In Builder.
7) Save your Add-In.
Please note that if you make any further changes to your add-in (in Add-in Builder), you will need to remove this file from additional files, save the add-in, and repeat the process above.
Check out this add-in for a list of icon references/names.
Justin