Each NIM100 has a coax in, a coax out, and an ethernet port. Think of one NIM100 as a bridge between your ethernet and your cable lines. So you would have a NIM100 hooked into your router (assuming it has more than one port in it since you also want your cable modem plugged into the router as well) or some other place in your ethernet network (I plug mine into a switch nowhere near my cable modem.) That same NIM100 would also have a coax line running from a wall outlet to the NIM100. The coax out is there to pass through in case you want a TV hooked up to it, etc.
If you want the cable modem on the same wall outlet, and assuming your cable modem doesn't have a pass through coax out, you'll either have to split the wall outlet using a passive splitter and run one to the cable modem and one to the NIM100, or you'll have to use the coax out from the NIM100 and run it to the cable modem. The splitter will introduce a bit of signal noise or loss or whatever that measurement is, which can affect quality of internet service. I don't know what, if any, effect running the coax out from the NIM100 to the cable modem will have.
Once that NIM100 is in place, you would put a second on any cable outlet in your home (avoiding active splitters as I discovered), again running a line from wall outlet to coax in on the NIM100, and then you are free to plug any ethernet device you want into the second NIM100 such as a switch, or a computer, or a MediaMVP
Of course, all of this discussion makes me wonder...how far outside of one's house does the information passed around actually travel? Could someone take a NIM100 and splice it into the coax line that runs from my house to some box outside and be on my network? Or worse, I know there's a box outside my house that essentially looks like a big splitter, where my cable line hooks up to some main trunk and my neighbor's does too. I assume it then runs to a larger junction box of some kind down the road where I saw them working one day when they needed to clean up the signal leading to my house to make my internet service work. Could my neighbor plug a NIM100 into their cable outlet and see my traffic?