In MacPractice, go to Preferences > Digital Rad. Bridges, and select the bridge you wish to configure. Then, click the Choose button in the bottom right corner to set your bridge import path.
Select your bridge import folder in the window that will open. The default path is /Users/NAME/Library/Application Support/MacPracticeClient/Databases/macpractice/WindowsBridge.
Some bridges will also have a "Settings" section where you can select the Application Path. Type the file path of the Windows DR application. In most cases this will be something like "C:\DEXIS" or "C:Program Files\DEXIS". If you are using Windows 7 in 64 bit mode, it may be "C:\Program Files (x86)" instead of simply "Program Files".