Hi @flashgordon ,
I assume you want to record the audio straight from your Korg X50 into Cubase.
I assume that if you connect the L and R cables into your Focusrite and play some note on the synth you should be able to see some indicator that the sound is passing trough.
I do not have a Focusrite card but generally it is matter in the Studio> Studio Setup (Note: may be a different menu in Cubase 5, it is where you setup your audio driver) menu to go in Audio System and select your Focusrite Driver.
After that if you press F4 to access the Audio Connections menu in the first TAB where it says input select for the Stereo In the Focusrite inputs.
If you go in the Mixer pressing F3 you should be able to see the Stereo In input indicator when you play a note.
If you want to record a track make a new Audio Track and if you hear no live sound press the monitor button (the speaker icon in the track)
Note: When you playback you need to disable it else you will not hear your audio.
I generally do like this make a Temp track with the monitor enable that Iuse only to listen when I record and then have a real record track that has no monitor but can record, so I can hear it when I playback