I've used the same installation method you mention to get 5.1 and 6.0.1 SDKs onto Snow Leopard. (Note that just installing the SDKs doesn't include the simulator, so it won't support 5.1+.)
I have Snow Leopard 10.6.8, and breakpoints fire fine, both on my updated phone connected via cable and in the outdated simulator. Should work fine on 10.6.7 also.
I would recommend that you create a new project and set a breakpoint. If that works, you are expecting the other project to reach a breakpoint but it doesn't, either because of the code or an XCode setting.