I installed Weaver Show Slider 1.2.1 and put up 1 Post with a Gallery (Works Great) -but- after building a second Weaver Slider Post with a Gallery it is not in the Add_Show_Slider List (only the first Post in List) – if I use the short-code to put it in I get “ERROR with [atw_slider name=”postpic”]: You must specify a valid slider name.” ?????
I think you are confusing the slider posts and the slider themselves.
Slider post only allow you to define the content for a slider.
In order to use that content, you need to create a Slider, and select the slider post as the content to be used for it, in the “Slider post slug” drop down list found in your new slider “Slider Options” box