Well then it looks like you're in some deep %^&* if you have no idea what you're doing.
I have never used those modems before, nor a ML1400 as a DNP3 slave (I will be testing this on the bench in a couple months time, but haven't used it yet). So I cannot possibly give you a step by step instruction guide.
However, I suggest the following:
Step 1: YOU draw a network diagram showing how you think your network will look. Include base SCADA PC, network equipment, etc. Include IP addresses, subnet masks, default gateways. All this is important to know before you start trying to configure stuff. Post that here so we can see if you have some understanding of how things are supposed to be working.
Step 2: YOU read the modem manual and figure out how to set up a VPN from your base station to this remote site. Normally this requires one side to have a fixed IP, i.e. your base station SCADA server would have a wired broadband connection, with a fixed IP address. Your base would also have a VPN appliance. The cellular modem (which will typically have a dynamic IP), initiates the VPN connection back to the base. Setting up a VPN that is actually secure is a whole different topic, but there is plenty of information on the web for at least getting something functional for now.
Step 3: Once you have your VPN correctly established (i.e. you can ping your remote cellular modem from the Base Station SCADA server end), then you need to get your micrologix connected via Ethernet. Set an IP address (that matches your drawing in step 1) and tick the "Enable DNP3 over IP" in the ethernet channel config. Set up at least one class of data types so you can poll something from the base. re-power the PLC for the IP address to take effect, and check whether you can ping the PLC from the base.
Step 4: Configure your SCADA server software for polling a DNP3 slave. I had a quick look at this KingSCADA (never heard of it before...) and they say they have over 3,500 drivers but don't list any of them which seems fishy. Does it have a native DNP3 driver? If not, I suggest you contact them and find out what 3rd party DNP3 driver they recommend. Then purchase said driver, and figure out how to set it up. It will probably be OPC, so hopefully KingSCADA supports that.
Step 5: Since none of the above has probably worked, give up for the day and head to the pub for several beers. Contemplate why on earth you signed up for a project where you have no clue what you are doing, and then start back at Step 1. Repeat until you either get fired, give up or actually get it working.
Good luck.