First, make sure your app permissions are properly set so the phone can get your location. To do that:

1) Go to Settings and make sure Location Services are set to "On".

2) Next, tap "Location Services" and make sure Foursquare specifically is set to "On."

If your phone is still having trouble finding your location, try these steps:

1. Quit Foursquare. (You will need to shut down the background instance of Foursquare. See here for info on how to do so.)

2. Open the Settings app.

3. Go to Settings > General > Reset > Reset Location & Privacy.

4. Restart Foursquare; your phone should now be able to discover its location.