Coding menu with dropdown/sub pages within theme…

I purchased and activated a particular theme, which is supposed to have a built in CSS dropdown menu. (HotGoss 2)

This does not seem to work, (does in their demo, doesn’t on my sites!) so I am attempting to ‘hard-code’ or add to the code (which I assume needs to be in header.php?) to create dropdown menu tabs which list subpages under each menu tab heading.

What I ideally want is each menu tab heading as the name of each parent page, but unclickable, and the sub-pages of each in a dropdown list which can be selected.

I am comfortable with inserting code, but I am so far unable to find a straightforward script or instructions on how to do this. There must be a standard way to do this, regardless of theme? Or am I wrong?

Any help gratefully received; apologies for newbie ramblings!