You do not say if you were installing on a dev or retail unit.I'Ve tried to install my Unity4WiiU-Demo I made, which contains a .tik, .cert and .tmd file (+ some h3 and app files) but I get the same error.
But as stated before, if you didn't downlaoad it from NUS Do NOT install it.
There is next to NO chance you have an app signed for retail.
There are NO disabling of signature checks currently. So your effort is most likely wasted.