I am looking for the “Surround Panner” Plug-In which was there in earlier Versions and now it is missing.
They changed the way it works. If you assign a mono or stereo track to a 5.1 output bus (that you have to set up in VST Connections) and make sure Surround Panner is present in the Inspector (right click somewhere in the Inspector and check it in the list) it will be (and this is what really changed from earlier versions of Cubase) automatically inserted into the channel. Technically, it is a plug-in and it is inserted into the channel…but not in the “conventional” way of inserting plug-ins (or like the way it used to work as a plug-in in C4).
If you have a mono or stereo track assigned to a 5.1 output and the Surround Panner is visible in the Inspector and you double click on it, it will throw up an editor similar to (but not the same graphically) as the old plug-in. The functionality is about the same as the old one, as far as I can tell.
Start reading from page 218 in your Operations Manual on the specifics.