If you are looking to use a remote, you will need the MC-DC2 wired remote, which plugs into the accessory terminal as stated in your user manual on page 177.
Or an IR remote that includes a receiver that plugs into the accessory terminal. (I am unaware of any, however, when I was searching eBay for the ML-L3, a two-piece remote popped up in the results, unfortunately, they gave no details.)
No, the D3100 does not have an IR transmitter built into the unit unlike it's predecessor the D3000 [yup, can't believe it either]. There is supposedly an after market remote that you can buy and adapt to the D3100 wireless. Cactus V5s or Yongnuo RF-602/603s.
I can't believe that this remote will not work with the D3100. I guess that I'll have to believe it. Bad, Nikon, bad. I just purchased the 3100 form Amazon, and now Amazon offers this remote to me as a "Gold Box Deal". Thanks for the info. I'm glad that I found this before making the purchase.