The Toolkit menus in Shotgun are managed by the environment files named
shotgun_<entity_type>.yml in your pipeline configuration (eg.
config/env/shotgun_asset.yml). Each of these controls the settings for the corresponding entity type you're viewing in Shotgun.
So for example, to add "Launch Photoshop" to the list of menu options you'd add an entry for your Photoshop Launcher instance under the Shotgun Engine apps section.
If you have edited the file and your item is still not showing up in Shotgun, see this page to update the cache.
If you're still not seeing your menu item, make sure that the
launch_photoshop section in
config/includes/app_launchers.yml is configured correctly for your platform. If there's no setting for your platform, the menu item will not display.