They may be there, just not when you're looking. Depending on the temps., they may be out in the early morning hours or close to dusk. Sometimes in milder weather [or after a rain], they'll be out mid-day. What is the physical set-up of your pond like? Younger boxies like more "cover" as it makes them feel less vulnerable to predators, so try to have plantings scattered where they can hide if they need to. Also, box turtles don't need deep water, so be sure there are shallow areas along the edge of your pond for them. Usually, they'll wander out into water up to the edge of their top shell, then sit & soak for a little while, or root around looking for something to eat. But they rarely seek water to "swim" in, so just be sure there are shallows for them.
You might try putting out some food for them [chopped apples, corn, bell-peppers....] a few times a week and see it that attracts any. Just have to be sure what else you might be attracting, so watch the food and remove it if necessary. Also, try to provide some nice soft areas that they can dig into, piles of leaf litter, etc. Mine love it when I pile old tree bark on top of some leaves -- they burrow down into that and even have a roof over them [the tree bark].
In general, boxies have their little areas that they routinely patrol, and they may have just not found your place yet. Give them time. BTW, this is the time of year for them to start slowing down for the winter, so your chances may be better come spring-time! Oh, and be sure your place is predator-safe: no big dogs, etc.
