How can I make my kefir milk smooth and creamy instead of watery and grainy?
I've been making kefir for about 2 years now and I've noticed a few things one of which is what you mentioned - kefir not thick and creamy.
I've found when you use too many grains that's what happens.
I make a pint (2 cups) so not as much as you but I only use two little pea sized grains.
If they start to grow larger than that, the kefir starts to come out thin.
Mine comes out nearly as thick as yogurt, but a little stir and it's drinkable.

The other person who responded is right as well, over fermentation: the result is little tiny curds. 

The more you make it, the more you'll find out how to get it just like you like it. 
For example I like mine really thick and also really carbonated and I've figured out if I put a tight lid on it (even though most sites say to only put a breathable top on it like a napkin or cheesecloth) when it's fermenting it comes out like a milk soda. :)

So maybe try only using 1 tablespoon for your quart (4 cups) and see if that helps?

oh yeah, and I use whole milk - using skim or low fat milk might make it thin.