Rich,
The lid should come right off after you remove the locking ring and open the air valve.
Did you try rocking it back and fourth?
I have used a wide common screw driver between the two halves on a couple of older DE filters, but I'm sure it is not recommend and not something I'd want to do on a new install.
There is a big O-Ring in there that could get damaged if you are not careful.
Thanks,
Jim R.