Dave you’ll need to be a little clearer on what your issue is?
It’s easy yo use the iRig cable with Cubasis. Just connect it as you would to use Amplitube. Run Cubasis. Insert an Audio track. Arm it and press the speaker icon beside the arm button to monitor what you are recording and your good to go as soon as you press record. Audio input in the inspector is set to mono input. You might also want to put the Amp Sim on as an insert effect.
If your asking how to record Amplitube directly into Cubasis you’re in for a disappointment as its not curently possible.
You can however run a loop you want to record over in Cubasis in the background by switching background audio on. I haven’t tried with Amplitube but most of the other guitar apps will happily run in the foreground and let you record to their own in app recorder. This can then be copied and pasted using Audio Copy Paste into Cubasis but you’ll have to aline it manually.
Hope this helps?