I am aware of no dictionaries with DRM (or from the Kindle store) that work as the primary dictionary on the Kindle. You can use one of the publicly available DRM-free MOBI dictionaries available here as the primary dictionary on the Kindle:
Webster, Noah: Webster's Dictionary 1913,
GNU Collaborative International Dictionary of English, and
Wordnet Dictionary for Mobipocket. I have only tested the GNU dictionary (not the other two), but I am anyway happy with the Oxford American so I have not switched.
I downloaded the sample of the Merriam-Webster's Collegiate Dictionary, 11th Edition on the Kindle, but this is not detected as a dictionary and isn't even usable as a conventional book dictionary. The entire dictionary might be ok though, and I'm sure you will get your money back if it does not work.