Hi Victoria,
you can add shortcuts to most menu items of (almost) any program in System Preferences > Keyboard > Shortcuts > App Shortcuts.
(That is, without the need of Keyboard Maestro.)
See also this:
For a more detailed description by Apple see here.
Have you already tried that?
Since I couldn’t find a Trial version of Family Tree Maker I cannot verify if it works for that particular menu.
Note:
You have to type the name of the menu item exactly (including case and punctuation) as it is displayed in your app. In case of the Merge Individuals item this would be, according to your screenshot:
Merge Two Specific Individuals…