What did you set the trial up on?
But to use the LG NowTV App, just open it on the TV, and set up the same account details in the app that you already set up elsewhere for the trial.
Please note that after 7 days, the free trial automatically morphs into a month-by-month subscription, unless you Cancel it earlier than that. NowTV try to make this clear, and I hope you are clear about it, but people still miss it.
So if you inexplicably decide that NowTV is not for you (!), then Cancel the trial on or before the 6th day. It will still run for the full 7 days, even if you do Cancel it early, on both devices. Though you only need to Cancel it on one of them,
OK, that’s fine.
Did you get the app going on the LG TV, so you can see the bigger picture?