So to this point I've read just about everything on 240v, 3 prong wiring and I'm still stuck.

I bought a 240v, 4800w 20amp garage heater. I added the 30amp circuit (noted on the box as the required circuit), ran the 10/2 wire (it happens to be 10 CU 2, if that makes a difference, I was told it didn't), connected the 6-30p outlet, tested L1-ground, and L2-ground, each at 120v and... the heater doesn't work.

I'm sure I'm missing something tremendously simple, but it's completely evading me.

Exact brand and model of unit

One quick check you can do. Use a meter and check between the two legs of 110 volts. Jump across the two legs of 110 volts and see if you get 220 volts.

It's a dimplex dch-4831a

Did you

Jump across the two legs of 110 volts and see if you get 220 volts.

Thanks hvac. I just got back from my panel, and adjusted the circuit . I had it in the wrong position only using one of the lines... that solved it.

That does happen. You just cannot get 220 when using the same phase side. Glad you got it going.