Since Apple removed the X11 support you may faced some problems with X11 based applications. Wireshark is one of those applications which made trouble on my mac. To fix the problem you need to install XQuartz.
Download the lates Wireshark version and run it. It should come up with and selection window for your X11 application. Click on browse and navigate to your application directory. Open the directory utilities and select XQuartz. Now quit Wireshark using cmd + q and start it again. It can take a second but now your Wireshark should work with Moutain Lion.