The Foursquare app for BlackBerry tries to locate you using GPS. If this doesn't work, we fall back to the location of your closest cell tower. In urban areas cell tower location is pretty accurate and less accurate in more rural areas. Of course, if the app doesn't list your current location as the closest one, you can always refresh the Places tab (press the menu button, then choose 'Refresh') and search for it.
We're always working on updating the BB app, but make sure you have the latest version of the app in the meantime! You can also check out our BlackBerry Help Guide.