I've searched and found advice to use a GIMP layers dialogue in e.g. How do you get transparent colour on Gimp? However, in my version of GIMP (2.6.8 for Mac), Layers doesn't seem to be set up to display a dialogue. I've seen it in previous versions but hunted and not found anything under the Layers menu that will bring up the Layers dialogue.
How, in the present version of GIMP, can I set the opacity image-wide to 30% or 70%?