Just made a simple rig and was watching 'building first rig' by HomeSlice.
Got to the point to do my Constraints, when I try to pic target bone it does not select the one I choose, so I go to select target
and the needed bone (s) do not show up in the list.
I'm sure it is something simple i have missed.

Did not look into your rig, but depending on the constraint you want to add, it may be that the bones have the wrong hierachical structure for the constraint.
Are they in one bone-chain?

See you
*Fuchur*

Also, if bones are hidden in the model, they won't show up in the dropdown menu.

The bones are not hidden and they are, if I understand right, not in a one bone chain. It is a simple straight forward rig with no fancy extra bones, some attached others not but for some reason I am unable to select the bone need for a constraint. Some times most of the bones will show up in the list of tatgets sometimes just a few will.
It's a mystery to me.

What constraint are you trying to do to what bone in this model?

As fuchur said, it's probably as hierarchy thing. From what I saw on your model, all the bones were unhidden, so that's not the issue. If you have bones occupying the same space, it will be tough to use the picker, but should show up in the menu, if the hierarchy will allow it.

As Robert asked, what bone to what bone are you trying to constrain.

head, orient like, neck or neck to head,
hand to forearm,

I used the mirror bones plugin and noticed in the Groups there is a
CPs without mirror
and
Unassigned CPs

would ehat have anything to do with it?

Mirroring the bones and having unassigned CP's wouldn't cause the problem, Walter.

If you add a null to the end of the forearm, move the hand out of the arm hierarchy (I would move it out of the entire hierarchy), and then have it "translate to" and "orient like" the target null, I think you'll be able to do a lot more with the hand...the same with the head (except the null would be at the end of the neck and the constraints would be to that null). I would also do the same thing to the neck (with a null on the end of "spine 2")...that would give you the ability to turn on and off whether the neck/head orients like the chest.

Hope that helps.