How can I record an instrument such as guitar or keyboard in Studio One? Not with a microphone but with a cable. I have a Rocksmith tone cable but would I need something like a MIDI
This will all depend on your hardware. For guitar you would connect to an audio port (or line in if you are amplifying before you get to your DAW platform) and for a keyboard you could use either audio or MIDI, depending on what you plan on doing.
The Rocksmith cable Is different again - it is a full audio to USB interface, and as long as you have the drivers, it can be used for your DAW.
Studio One can cope with all the audio or midi interfaces you are likely to come across, but this is dependent on the hardware platform you are using.