Peat moss is very hard to saturate, and it's best to wet the mix as it's being put into the box. Even when a good saturation point is achieved, the top will become dry when exposed to wind and sun. I live in a location that is very sunny/hot in the summer, and love Mel's mix. Of course - I only add water to the area of each plant, and not the entire box.

EG

Here is a link that might be useful: EG's Garden Blog

This is my first season of SFG and Mel's Mix, and one thing I notice is that the top 1/8 inch of soil dries out super-fast, but scratch that away and the rest of the soil is still damp. I'm not sure if that is happening with your mix, but it certainly happens with mine. (also: I'm not technically using orthodox "Mel's Mix". I'm more in the neighborhood of 25% peat, 25% vermiculite, 50% compost. Maybe that makes a difference.)

Here is a link that might be useful: Luke's SF Victory Garden

My mix is similar to Luke's in that it is heavier on the compost than the other ingredients. I notice the same thing. It looks dry, but when I dig a bit into the soil it is damp.
I probably don't water enough, but I forget to do it! Even on my skimpy watering last year I had some good harvests.

Maybe I didn't scratch at the soil enough. I will check it out again tonight and get back. I was wondering how well seeds germinate in the soil when it gets so dry on the top layer.

I thought the same thing after a big storm. The top was dry, but I dug down and it was really wet at the bottom. I put plywood on the bottom of my box, so that could be why, but I checked the next day and it was wet uniformly throughout the box.

Mel's mix is designed to saturate the vermiculite, and to drain the remainder quickly.

Try squeezing the vermiculite in that "dry" mix. The grains hold water for a very long time, and the roots can take it straight from them.