Demand5 doesn't show in the Streaming Channels store on the NOW TV box so you will need to do a factory reset of the box, which is a bit of a faff as you have to enter all your details again but if you want to then ...
From the NOW TV homescreen, go to Settings, then System, then Factory Reset and finally Factory reset everything.
once you've pressed the Play/Pause button three times as prompted your box will reset itself. You'll then have to re-enter your wifi network details and your NOW TV account details, but at the end of it you should have Demand5 back again.
Do you mean the app has now installed but it's not working properly, or the app hasn't installed?
I've checked it on my box (deleted the app, factory reset the box to reinstall) and the Demand5 app downloaded fine during the setup process, so it might be worth trying again.
Make sure you do a factory reset, not just a restart as that won't do anything. The other way to do a factory reset is to push a pin into the reset hole in the back of the box while the box is on, and keep it pushed in until you see the box restart itself.
If the app has appeared but isn't working, try to describe what the problem is and someone might know how to solve it.
Might also be worth checking on the following list to see if you have any other devices that you can watch Demand5 on
Had problems in last few days loading Demand 5 due to server issue at D5. Decided to go to Options and remove app with aim of reinstalling from Streaming channels. Removed app but now can't find demand 5 from app list. Can anyone advise?
Thanks very much
Welcome to the forum.
You are right, Demand5 doesn't show in the Channel Store, but if you do a factory reset of the box it will be redownloaded as part of the setup process.
With the box powered up, push a pin into the reset hole in the back and hold for a few seconds until you see the box restart itself (or use the Factory Reset option in the Settings-System menu on the homescreen). Once the box restarts you will need to enter your wifi and NOW TV account details again, and then Demand5 should be installed along with the other default apps.