Not adding the DME means that your beer had a lower gravity than intended.
With just the 6.6# LME, the OG would have been around 1.055.
With the extra 1# if DME, the OG would have been around 1.064.
So, your beer is lighter version of the original recipe. 1.055 is still a pretty good strength beer (most of the beers I make are in the 1.050 to 1.055 range), so it's not like you made some watery pee-pee or anything. It's just not going to tank you as fast.
You *could* still add it (boil it up in some water first), but it will definitely kick up a harder ferment if you do.
I personally wouldn't bother adding it and would just let it ride. But, if you DO add it, I'd have a blow-off tube rigged up just in case.