I am able to reproduce the error message "The selected application is not regognized as a valid real-time decoder for Spirent TestCenter" when I try to configure Tools - Option - Capture - Real-time and try to attach the wrong exe file (for example explore.exe)
Spirent engineers currently write our own extension to specific Wireshark versions that does real-time capture and interpret Spirent signature.
For ths reason, for real-time capture, it is required to use the supported wireshark version available on the Spirent Customer Support web site http://support.spirentcom.com/ (requires login) -> Downloads -> Tools