OK, This link should work:
https://apps.facebook.com/twitter/?ref=tsif not, just log in to facebook and do a search for "Twitter" in the same way as if you were searching for a person, then follow the instructions. You need to change some settings on your twitter account, but it tells you what to do simply enough.
If you have a business & a personal FB page, you need to check the right box on the Twitter settings to make sure your tweets get posted onto the correct one.