background.js

chrome.browserAction.onClicked.addListener(function(tab) {
  alert('Oh hey there! You checky clicker!');
  chrome.tabs.executeScript(tab.id, {code: 'alert("I just came from the page!")'});
  chrome.tabs.executeScript(tab.id, {file: 'some_file.js'});
})

manifest.json

{
  "name": "Clicker MoBob",
  "version": "1",
  "manifest_version": 2,
  "description": "Calls you mean thinks when you click the button",
  "background": {
    "scripts": ["js/objects.js","js/background.js"]
  },
  "default_locale": "en",
  "content_security_policy": "script-src 'self'; object-src 'self';",
  "permissions": [
        "tabs"
  ],
  "browser_action": {
    "default_title": "Click me",
    "default_icon": "imgs/icon.png"
  },
  "icons": {
     "16": "imgs/icon.png",
    "48": "imgs/icon.png",
    "128": "imgs/icon.png"
  },
  "minimum_chrome_version": "18",
  "offline_enabled": false
}

some_file.js

alert("I'm the file you injected.")