I need all the visible text to be ( a similar thing to Helvetica)

I checked your blog and I found that you have selected the FF Dagny font for all of your fonts on the Appearance → Custom Design → Fonts page.

If you were to switch back to the advanced fonts mode (the one where you have to enter your Typekit Kit ID), then I would recommend looking at the original theme stylesheet to find all of the selectors the theme uses to set fonts. You'll want to use those same selectors in the editor over at Typekit.com.

Here is the original stylesheet link for the Forever theme in case you still need it:
https://s1.wp.com/wp-content/themes/pub/forever/style.css?m=1344521605g&minify=false

Note that using the CSS Editor would be another alternative if you wanted to actually switch to Helvetica. You could do something like this:

body,
button,
input,
select,
textarea,
h1,
h2,
h3,
h4,
h5,
h6,
#site-title,
#site-description,
.recent-post .entry-meta,
.page-title,
.entry-header .entry-meta,
footer.entry-meta,
#comments-title,
.comment-meta,
#reply-title,
.widget-title,
#main div.sharedaddy h3.sd-title {
	font-family: Helvetica, sans-serif;
}

Those are all the selectors from the stylesheet I linked above that have "font-family" set for them.

Does that answer your question?

I have purchased the Customisation Upgrade and have successfully added my Typekit ID, unfortunately I do not know what 'selectors' I need. I have picked one font, in which I need all the visible text to be ( a similar thing to Helvetica) and do not know which selectors to use in my Typekit Editor.

The page is http://tedxlittlelonsdalestwomen.com/

Thanks