Ah, I see how they're doing it. That volume lug is normally "bent back" to touch the casing, in order to ground it. So, that's the volume pot's "ground". Possibilities:
1. You're not getting continuity between that volume lug and the casing.
2. If you DO have continuity, as per #1, then your volume casing might not be grounded. Does the volume work? The "Black wire from output jack" has to be soldered to the volume pot casing.
Or to put it another way, here's your tone capacitor's signal path as it currently stands:
Tone lug --> volume lug --> pot casing --> ground
Since it ultimately goes to ground, you can cut out the middle man by just wiring the capacitor right to the volume pot casing and making sure that the volume pot casing IS going to ground ("black wire from output jack").
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Additionally, that wiring scheme would make for a very straightforward conversion to "star grounding". I mean, if it ain't broke, you don't have to fix it, but when I change electronics, etc., I always take the opportunity to make a star ground while I'm at it. If you want to give it a try, I'll hook you up with the knowledge needed. Otherwise, try my above advice and see where it gets you.
Greg