cannot activate sensor with iphone 3gs

I had the same issue with my sensor at first. I don't know what to tell you that will be efective but just to stay with it. I did the following things to finally get mine to sync (I'm not exactly sure which one did the trick.) I did a hard reboot, I turned bluetooth and wi-fi off (I know that's not how the sensor works, but I didn't want any interferance), I turned location services off, I walked around in circles for more than ten minutes, I held my phone as far down as I could... I know that's a lot, but it finally worked and now that it's synced, it works great.

Good luck with it, I know it's frustrating... I know I was when it happened to me.