This is a paid extension so I can't get it to verify. Please go to Creating and validating XML signatures | Adobe Developer Connection and install the validation tool to validate the signature of WebZap extension. Unzip this extension to a empty folder with 7-zip or winzip, then click "Load signature" button in the tool, browse to META-INF/signatures.xml to load it, then click "Verify signature" button.
Extension Manager CS6 uses the same method as this tool to validate extension signature. If this tool says the signature is invalid, then EM CS6 will also treat the signature as invalid.
It's also possible that your environment affected the signature validation. For example, the account name contains special characters, some root certificates are missing in your machine, timestamp server or CRL server is not reachable due to your Internect connection or firewall, etc.