Toolbar position relative to active menu item

TL;DR: Grouping related items is usually a good thing.

If we look at Whatsapp (a popular chat app currently owned by Facebook) as an example and examine how it handles 'action' placement, you'll notice that it's all on the right. This definitely has to do with the fact that it's a mostly mobile app (though the web variant does the same). But this is also good for new users, since they don't have to look far to know where the actions are (e.g. adding an attachment and recording audio are right next to each other).

Now, how does this relate to your app? You could do the same and place both the action and the content menu next to each other (vertically aligned). It can be placed on the left or, as shown below, on the right. but I think this will be a much better experience for your users.

I have an alternate suggestion, which may or may not make sense based on how you're adding new items (and how you're displaying them in the Content Menu).

If those Content Menu navigation items are showing those types of content when clicked, you may want to put the "Add New" button alongside each menu item in the content menu. This is a common pattern and tends to be clear, since you're clicking "Add" alongside the very kind of item you're browsing to in the content menu. Here's a quick mockup in case this is unclear (the red "Add+" buttons showing where those might appear):

Since the user would be adding a Content, it would work best to add the button closer to where the action takes place.

For this reason, your second option is preferable "In the right of the content area, because you add content."

When you think about it, you're already applying the same logic looking at how you've nested the different menus sub-layers. It works great despite the amount of content you could potentially show.

Good continuation on your project.

I think the solution should depend on whether selection of the menu option acts as filter (or tabs) or not.

If menu acts as a filter or tab, e.g. user sees the list on media, when "media" option is selected and nothing else, then I would go with second option but with left aligned actions. Moving important actions to opposite side of the screen is not helpful for users.

If option selection does not change the content area, then consider using accordion control to house content types specific actions if you think that there will be more then one action or controls for each content type. Proximity of content menu options and related actions is good for your users.

Otherwise, if user can do with content is to add new object into content, then content menu options itself should add new object, or get rid of the context menu add "New" drop-down list to group menu.

I think a layout that would be more friendly is an option section with an image of what content they are adding with the addition button next to that image. I have added a concept below.