To up/down vote, submit, or save posts to your profile you must be logged in with a reddit account.
To log in on iPhone, tap the bacon icon and then the tap the gear icon (Settings) in the lower left corner of the screen.
To log in on iPad, tap the gear icon (Settings) on the left side of the screen. If you don't have a reddit account, tap on the Create Account tab that appears after you open the login screen.