What is the hotkey to show Chrome extensions?

There is none built-in; you'll have to install an extension specifically for this such as "Extensions HotKey" or use a shortcut/hotkey management extension (like "BookmarkBar HotKeys") and assign the extension page (chrome://extensions/) to a key combo.

• Right-click in the Chrome address bar
• Select "Edit search engines..." from the menu
• Scroll down to the bottom of this new window where you'll find a row to add a new entry. Where you see:
  • "Add a new search engine" enter any name (e.g. "asdfgh")
  • "Keyword" enter your desired shortcut (e.g., E )
  • "URL..." enter "chrome://settings/"
• Select "Done"
• Now you can get to the Chrome Extensions page from within Chrome by entering CTRL + L, E, Enter

On Mac OS X, there is none by default, if you mean Window » Extensions. Just add one in System Preferences:

Result:

Or you can always enter (and even bookmark) chrome://extensions/ to get there.