There are 2 issues here:
1) Displaying the sidebar in a local prototype
The sidebar should be shown by default as long as you load the correct page. Make sure the browser is loading the root directory or the pages start.html or index.html. As in:
2) The need for the Chrome extension
There's unfortunately no way round this for local prototypes. The extension is required to counteract Chrome's in-built security feature that prevents the user running local content that may be malicious. If your client can't install the extension, you'll have to run the prototype from Axshare or your own web server. The extension isn't needed to view online prototypes.