Yes. You need the 64bit addons as MC is 64bit.
As mentioned, Sharks + 64bit addon will work and is a good one.
Depending on what you need, DivX Codec Pack works for MKVs in 64bit as well and does well.
Many prefer Sharks over all else, as it handles everything. DivX will not handle as much but will do MKV in 32/64bit. This would be a minimal codedc package in comparison.
Whichever you choose, only use one or other or you'll end up with problems.Same goes for any codec package.
If you want to try a new one, uninstall all of the old first. Sharks probably is the best bet if you want to cover absolutly everything.