What version of WinZEC are you running? Assuming that everything is fully synced as mentioned above and the key was correctly imported the first thing I would try would be starting zcashd with the rescan flag. According to the docs:
-rescan
Rescan the block chain for missing wallet transactions on startup
This can take a while so just leave it running. You’ll need to interact with the underlying zcashd instance rather than the WinZEC frontend. So shut everything down and browse to c:\Program Files\winzec\app (default install location) and then open a command prompt there and enter zcashd.exe -rescan. Let it finish and fully sync and then you’ll then be able to open WinZEC and hopefully your funds will appear.