To sign in with Read&Write for iOS with a Read&Write subscription, open the Read&Write app on your iPad. 
Tap Start setup wizard button to activate your keyboard:
Once your keyboard is set up, Tap Next Step to register your account:
Select Yes button if you have an existing account. (You can still access non-premium features for free if you are not a Read&Write user).
Enter your Serial or Product Code to link your account, then tap the forward arrow 
Or use the Sign in Option, You would be prompted to select your account provider: 
That will link the account to allow the premium features.
If you've done this and are still not seeing the premium features, please click below for further instructions.
NOTE: You can still access Read&write for iOS if you are not a Read&Write user, it would prompt you to start a free trial
I get the message 'Please enable full access to use this feature'