This is an issue caused by the recent iOS 4.0 update. Voxie 2.9 fixes this, so please update. BUT if you have 2.8 and are looking for a workaround (which we don't suggest), see below:
That said, you can go to your device settings, tap on WiFi, Tap the blue arrow next to the name of the WiFi network you are connected to and read the number in the "IP Address" field. Then take this number and type it into your browser as follows (the X's represent the numbers of the IP Address).
Remember that you need to have Voxie Sync running when you enter that URL (address) into your web browser.
And remember that this number can change any time your network decides to change it. So you should check to make sure what your current IP Address is each time, just in case.