I've searched and found advice to use a GIMP layers dialogue in e.g. https://graphicdesign.stackexchange.com/questions/8898/how-do-you-get-transparent-colour-on-gimp/9071#9071 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%?