Holding the remote in your hands with the standby button pointing away from you, push back on the black half of the remote to separate the upper and lower halves of the remote. Slide the battery out and replace only with the same type, CR2025 3v.
Check if your WiFi router is WiFi 802.11b/g certified. The A100 and S300 is not compatible with N Class routers. If your WiFi router is WiFi 802.11n, please ensure that it is in 802.11b/g mode. The A100 and S300 is not able to talk to routers that are 802.11n only.
Move the A100/ S300 closer to your WiFi router and avoid using a WiFi channel which is overlapped by the nearby WiFi network. Also check if your network speed is fast enough (in cases where multiple devices share the same router, the AirPlay performance may be affected). Turn off the other devices connected to your router.