Read the activation guide.
If yours does not come with one here are the cliffs notes....
1. Activate LTE service - Call 877.807.4646 from a landline preferably and follow the prompts. You'll need the receipt that came with your bionic
2. Insert the SIM card and battery. Put the battery cover on the phone and power it up.
3. "Activation will begin automatically. If prompted, follow instructions to accept customer agreement"
4. Follwo on screen prompts to setup your phone.
The actual 4G synching of the phone to the network can take some time. There will be a message that says something along the lines of "Activating your phone, please wait while we activate and reboot your device" (NOT exact wording at all). Just let it do it's thing. Mine took about 20 minutes to finish.. other have taken hours. Relax, get a snack, do something else... your phone will connect to the network eventually.