The release is a bit staggered so that some locations have it, some don't have it yet, and some don't plan on having it at all. Your best bet is to just inquire at your local McDonald's whether they plan on offering it.
In case you've never heard of a McRib, it's a ground pork patty formed to slightly resemble a small rack of ribs and comes covered in barbecue sauce accompanied by slivered onions and pickles, in a long, cornmeal-dusted roll. If you wondering how it tastes, you can find my review of the McRib here.
Prices vary with location but tend to fall in the low to mid $3 range.
Calories - 500 (from Fat - 240)
Fat - 26g (Saturated Fat - 10g)
Sodium - 980mg
Carbs - 44g (Sugar - 11g)
Protein - 22g