I need to pour a 5 or 6 inch tall   "wall" ( for lack of a batter word) around the outside of my existing  carport slab for an addition I'm building.
I want to make sure I  keep moisture out. I had moisture problems  with the last wall that sat directly on the slab due to rain coming off the roof and splashing back. Yes I have gutters, my roof is very large.
How can I " join"/seal the new concrete to the old?
Thanks in advance,

hammer drill some lags, install some rebar, and I believe there is an epoxy-esque substance you can put down in between, I forget the name/brand, but hopefully someone will chime in with the answer.  After it's all done, seal coat it.

You should be able to dig down around your slab. Here in the New England you would want to put a strip footing around the slab below the frost table. @ 48"

The fact that you are going to pour the new concrete tight to the old should be a good start, you can use the edge of the slab as part of the formwork.

You could actually dig it so that the concrete actually wraps around the existing slab edge.

When in doubt use some Liquid Nails

Liquid Nails

You mean curb? I would first roughen the old concrete surface to provide something for the new concrete to adhere to.

Get some #3 rebar and cut it into 8" or so lengths, drill holes about 18" apart in the old concrete about 1/8"diameter bigger than the #3 rebar.

Make sure you leave at least 2" from the bottom of the existing concrete when you drill the hole.
IE if the exist concrete is 6" thick, only drill 4" down.

Also, leave 2" cover of concrete above the top of the vertical #3 bars glued into the exist slab. IE, 6" curb, leave 4" sticking out of the existing concrete.

Fill the holes with concrete epoxy such as Ramset C6 fast cure or Hilti. Stick the 8" long rebar into the holes and wait about an hour to cure with a fast setting epoxy like the C6.

Tie one continuous #3 rebar at the top of the vertical rebars glued into the the existing concrete.

That should help.

You need more downspouts too.

rj