How the feck do I add new bars to my existing neighbourhoods?

Step one: enter Edit Town mode from your current neighborhood.

Step two: Look at the sizes of the bar lots in your "place community lots" selection tab

Step three: Either using world edit to place a new lot to match or exceed the size requirements of the desired bar lot, or locate an existing empty lot in your town that meets the lot size requirements

Step four: Select the newly placed/located lot and change lot type to Community Lot then select the appropriate community lot sub-type from the drop down menu to match the bar you're wanting to place. (Dive bar, lounge, sports bar, dance club, etc) 

Step five: select and place the desired bar from the Community lot list on the newly created lot OR edit the new lot and design your own bar, checking the requirements to ensure it will open for business when you return to game mode.

step six: save changes and return to game 

Hope this helps.