Just to explain fully, cause it's really half hidden:
The Player arbitration screen is only visible on Arb day, click on Arbitrate to get to it.
There's a paragraph in the middle of it that explains how arbitration works, then at the end there's a final sentence that says something like "or, you can sign him to a long term contract."
That portion of the sentence may be slightly bolded, depending on your browser display. If you hover your cursor over it you'll see it's actually a link. Click it and you'll go to the Contract Extension screen just like before Arb day. It's the last chance LT link.