Not sure about your specific phone but I have a Moto X and the process for that one is:

1. Tap on Settings
2. Tap on "more" under the wireless & networks grouping
3. Tap on Mobile networks
4. Tap on Network operators (choose a network operator)

From there, each available network is listed and I simply select the one for the locale where I happen to be from their list when they send me a welcome text (Unicom in the P.R.C. and Docomo Group in Japan). Of course, they may have more than one carrier from which to choose. Once the appropriate carrier is selected, it should register your SIM to that network.