All website/stores that were created before the Blog feature went live, will not have the blog automatically appear within their site navigation. This is partly because they may already be linking to their external blog (which is set in the Marketing > You On The Web section).
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In order to link your SupaDupa blog to your main navigation go into your back office and click on Settings> Main Menu > click the pencil to edit the Main menu, and then edit the destination for your chosen link or add a new one. From the destination, you'll want to select the 'Blog' entry in order to link to your SupaDupa blog.
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These screenshots may clarify:
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In your back office click on Settings > Menus > Edit Main Menu
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Choose Blog from the drop down menu to link to your SupaDupa Blog
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The blog now appears in your main navigation. Here we are using the theme Sidekick.