Apple's latest Safari Update (ver 7.1) automatically disables all installed extensions. You'll see a message that tells you that "Your Safari Extensions Have Been Migrated," but actually what Apple means is that they've all been disabled. Fortunately, it's easy to re-enable PriceBlink (and any other extensions you're missing).
- Click on "Safari"
- Choose "Preferences"
- Click on "PriceBlink"
- Make sure "Enable PriceBlink" is checked
- Close the preferences pane