1. Go to your Macro Settings(Where you can edit your keys)
2. Go the Events Tab(Could be named "Macro Bindings: Events" or something along that)
3. Click on "onItemDurabilityChange"
4. Click Edit File under Actions
5. Create a new file with any name
6. Paste in the text into the file
$${
IF(DURABILITY <50);
    DO(1);
        ECHO("/fix all");
    LOOP();
ENDIF;
}$$
7. Click Save
8. Type in $$<"The Name of the File you created".txt>
9. Click Save and you're done