The way to scroll using the arrow keys in a terminal emulator is to hold down the shift key. This is because a terminal emulator send all keypresses to the application running in the terminal; holding shift down bypasses that. This can also be used with other keys like copy and paste.
For those complaining that the shift key is unnecessary, changing the behaviour of the terminal emulator to not need the shift key would break all terminal applications that can make use of the arrow keys such vim, nano, git, less and many more. In all of those applications, using the arrow keys will just scroll as a user would expect.