Root [AT&T] Do custom ROMs not come with root?

[moeburn]

So I just installed SlimROM onto my S3. None of my root-apps can get superuser access. No superuser prompt shows up, they all get the message "Superuser access is needed but could not be acquired". There *is* a Superuser thingy in the Advanced Options, so I don't know why it isn't working. I tried installing SuperSU from the Play store, but it failed to "update the superuser binary".

What am I doing wrong?

 

[Madbat]

Did you open the superuser in settings and update binaries? Or in recovery (I use philz) there is an option to install su binaries.