The Boxee remote app turns your iPhone/iPad into a Boxee remote control and is available for free at the iTunes App Store.
Make sure that the web server is enabled and the port is set to 8800 (default) on Boxee under Settings > Network > Servers.
Make sure your iPhone/iPad is connecting to the same wireless network that the Boxee Box is on. Note: If your smartphone is connected over 3G or 4G instead of to your home wireless network, it will not be able to connect to Boxee.
Use Safari on your iPhone/iPad to call up the Boxee's web-based remote interface by typing the IP address of the Boxee Box and port number (e.g http://192.168.2.1:8800). If that doesn't work, then your iPhone/iPad may not be on the same network as the Boxee Box.
Make sure the web server password is empty under Settings > Network > Servers.
Android Remote Apps
There are a number of Android remote apps available in the Android Market including: