You're not alone, it is a very harsh and bitter beer. Especially when young. Sometimes even when aged.
Some things that I found to help:
1.) Control your fermentation temperature. Both in the primary and in the secondary. That means it has to be on the low end of the yeast temp range around 64F - 65F for the DanStar Windsor Ale during active fermentation. That's the temp of the wort not the outside of the container.
2.) Cold steep your dark grains. (Not sure this is possible with an extract kit; or even all grain; you probably have to buy all of the grains separately.) http://hbd.org/clubs/cascade/public_...ing/index.html
3.) Hot steep your crystal malts w/ a little base malt to eliminate residual starches (hold at 150-155 for 30 min.). http://www.homebrewersassociation.or...hp?topic=378.0
4.) Properly rehydrate your yeast. http://www.danstaryeast.com/sites/de..._datasheet.pdf
5.) Your wort/beer should be held at 64F-65F for a minimum of 8 weeks. Cold crash for 2 days; Now keg and wait 4 weeks; holding it @64-65F; sampling once a week until the flavors have mellowed.
6.) Add your DME at the start of the boil (preferably to cool water while heating the water) and the LME about 10 - 15 min before the end of the boil (I think they've updated their instructions to include this.)
7.) If your finished beer is darker brown/black instead of a lighter brown then you've caramelized or burnt sugars and probably won't like the result. The color should be light brown, not dark brown or black.
8.) Cut your water with distilled water. Use half distilled (or RO) and half tap water.
I've successfully made Caribou Slobber; but I had to buy all of the ingredients myself and put the kit together such that I could do all of the above.
It is a good beer when properly made and aged (mellowed); however, Northern Brewer marketing will make you believe that you can just "make the kit" per their lacking directions and the beer will turn out great in a short period of time! Unfortunately I was also mislead by this and the amount of work and knowledge to make a good Caribou Slobber is simply beyond that of a first time homebrewer.